Merge #296: Remove nixops examples and as recommended deployment method; replace with krops

520a983 docs: nixops-krops migration guide (nixbitcoin)
7c0b521 docs: describe deploying with krops in installation tutorial (Jonas Nick)
eb6dff5 docs: various improvements to installation tutorial (Jonas Nick)
e8b47f0 examples: add deploy-krops.sh (Erik Arvstedt)
fe118b2 examples: add krops deployment method (Jonas Nick)
e017675 krops: add package (Jonas Nick)
03db1a6 lnd, joinmarket: don't write to secrets dir (Erik Arvstedt)
55d8749 secrets: make configuration more robust (Erik Arvstedt)
b0c223e secrets: don't stop services when the secrets target stops (Erik Arvstedt)
b701cb5 secrets: add option 'generateSecrets' (Erik Arvstedt)
03515a8 examples/qemu: disable hardened preset (Erik Arvstedt)
908af3b examples/deploy-qemu-vm: show progress when waiting (Erik Arvstedt)
ccba86a examples: extract common features for running QEMU VMs (Erik Arvstedt)
cb3d544 examples: use relative paths to example files (Erik Arvstedt)
a4dabc7 Remove nixops examples and documentation (Jonas Nick)

Pull request description:

ACKs for top commit:
  erikarvstedt:
    ACK 520a983

Tree-SHA512: 380fcda3528e703e8a218f92a216befbee6cc279952388b2df48c1bcb803b72336bbe6e4deed87f3f26caba7a5cb5ab60c2bcabf2b13eba56f34c47eb02e128a

Author: jonasnick

.gitignore

@@ -1,33 +1,3 @@
-* **Q:** When deploying or trying to SSH into the machine I see
-    ```
-    bitcoin-node> waiting for SSH...
-    Received disconnect from 10.1.1.200 port 22:2: Too many authentication failures
-    ```
-    * **A:** Somehow ssh-agent and nixops don't play well together. Try killing the ssh-agent.
-* **Q:** When deploying or trying to SSH into the machine I see
-    ```
-    [email protected]: Permission denied (publickey,password,keyboard-interactive).
-    ```
-    Make sure you don't have something like
-    ```
-    Host *
-        PubkeyAuthentication no
-    ```
-    in your ssh config.
-* **Q:** When deploying to virtualbox for the first time I see
-    ```
-    bitcoin-node> Mar 19 09:22:27 bitcoin-node systemd[1]: Started Get NixOps SSH Key.
-    bitcoin-node> Mar 19 09:22:27 bitcoin-node get-vbox-nixops-client-key-start[2226]: VBoxControl: error: Failed to connect to the guest property service, error VERR_INTERNAL_ERROR
-    bitcoin-node> Mar 19 09:22:27 bitcoin-node systemd[1]: get-vbox-nixops-client-key.service: Main process exited, code=exited, status=1/FAILURE
-    bitcoin-node> Mar 19 09:22:27 bitcoin-node systemd[1]: get-vbox-nixops-client-key.service: Failed with result 'exit-code'.
-    bitcoin-node> error: Traceback (most recent call last):
-      File "/nix/store/6zyvpi0q6mvprycadz2dpdqag4742y18-python2.7-nixops-1.6pre0_abcdef/lib/python2.7/site-packages/nixops/deployment.py", line 731, in worker
-        raise Exception("unable to activate new configuration")
-    Exception: unable to activate new configuration
-    ```
-    * **A:** This is issue https://github.com/NixOS/nixops/issues/908. The machine needs to be rebooted. You can do that by running `nixops deploy` with the `--force-reboot` flag once.
-* **Q:** I'm deploying to virtualbox it's not able to connect anymore.
-    * **A:** Maybe the IP address of the box changed. Run `nixops deploy --check` to update nixops with the new IP address. Sometimes you need to run `nixops modify -d <deployment> network/network.nix network/network-vbox.nix`. Sometimes you also need to remove the old IP address from `~/.ssh/known_hosts`.
 * **Q:** The clightning service is running but when I try to use it (f.e. by running `lightning-cli getinfo` as user operator) all I get is `lightning-cli: Connecting to 'lightning-rpc': Connection refused`.
     * **A:** Check your clightning logs with `journalctl -eu clightning`. Do you see something like `bitcoin-cli getblock ... false` failed? Are you using pruned mode? That means that clightning hasn't seen all the blocks it needs to and it can't get that block because your node is pruned. If you're just setting up a new node you can `systemctl stop clightning` and wipe your `/var/lib/clightning` directory. Otherwise you need to reindex the Bitcoin node.
 * **Q:** My disk space is getting low due to nix.