Firecracker v0.13.0

Added

• Documentation for Logger API Requests in docs/api_requests/logger.md.
• Documentation for Actions API Requests in docs/api_requests/actions.md.
• Documentation for MMDS in docs/mmds.md.
• Flush metrics on request via a PUT /actions with the action_type
  field set to FlushMetrics.

Changed

• Updated the swagger definition of the Logger to specify the required fields
  and provide default values for optional fields.
• Default seccomp-level is 2 (was previously 0).
• API Resource IDs can only contain alphanumeric characters and underscores.

Fixed

• Seccomp filters are now applied to all Firecracker threads.
• Enforce minimum length of 1 character for the jailer ID.
• Exit with error code when starting the jailer process fails.

Removed

• Removed InstanceHalt from the list of possible actions.