Optional keys in favour of bootstrapping

[sfuerte]

• made the required keys optional, in favour of providing corresponding key_path variables during instance bootstrapping.
  Reasoning: when building an image, whether Docker, AWS AMI or anything else, it's better not include any sensitive information but rather include it upon container/instance bootstrapping.

• minor reformatting

• added a warning about not using variables invoking command line arguments with sensitive secrets, i.e. keys, passwords, etc. Use corresponding environment variables.
  Reasoning: leakage and exposure to procps and system logs.

$ ps axf  | grep concou
 365430 ?        Ssl    0:01 /opt/concourse/bin/concourse web ... --encryption-key <unencrypted key here>  .. --postgres-password <plain-text pass> ... --github-client-secret <plain-text token> ...

and here is an example of exported logs