Optional SSH known host keys configurable per project

[mumoshu]

I like the automatic and out-of-box support for SSH host key verification of GitHub and Bitbucket attempted in #86.

However, that isn't a generic solution as:

• It doesn't support other git hosts like e.g. GitHub Enterprise
• It is vulnerable to future changes in the host keys

Instead, how about making it configurable via project's values.yaml like the below?

# OPTIONAL: Set to whatever content you like to have for .ssh/known_hosts on worker nodes
# The below example was generated by running `ssh-keyscan -H github.com`
# When omitted, ssh host key verification is disabled(this is the previous behavior)
sshKnownHosts: |
  # github.com:22 SSH-2.0-libssh_0.7.0
  # github.com:22 SSH-2.0-libssh_0.7.0
  |1|a62YvGNxzDMqnXWOroqxhzaSflk=|yqCMdMiP53NP+vwrOATvJMvipTw= ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ==
  # github.com:22 SSH-2.0-libssh_0.7.0

[mumoshu]

Alternatively, we could go further with a much richer configuration API.
An example would be something like below, which we can see in an another CI system.

sshKnownHosts:
- hostname: github.com
   fingerprint: nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8
   type: rsa

I don't find any concrete use-case for such a rich API myself though.

[technosophos]

Yeah, I like the idea of making it possible to specify the key data like that.

[technosophos]

Somewhat related to #407

[krancour]

Closing due to staleness. Please do re-open if appropriate.