Consider supporting base64 encoded values for GIT_SSH_KEY

[JohannesRudolph]

Configuring the ssh key correctly is a frequent source of debugging pain when bringing up a new instance of unipipe-service-broker. This is because the key formatting is very subtle (preserve only some spaces, replace newlines etc.) and easy to get wrong.

It would be easier if we just supported taking the value as a base64 encoded value, so that operators can just get the right value via

 cat ~/.ssh/id_rsa | base64

and that takes care of converting it to a "configuration management friendly" single line value.
To make this change backwards compatible we can detect if the supplied config value is a base64 value (iirc it will have a = as the last character) and then convert, otherwise treat it as a key in the old config format. Going forward we would only document/recommend the new key format.

Another problem that frequently occurs is PEM vs SSH key format, maybe we could consider automatic conversions in a similar way?

Support for the old format could eventually be removed with the next major unipipe release.