Configurable optional fallback default remote repository

[neechbear]

https://github.com/nokia/gitcache-ssh/blob/master/gitcache-ssh.go#L65

defaultRemoteHostname is currently hard-coded. Perhaps we should not define a default at all?

If we do, it should at least be configurable from either an environment variable or from some kind of configuration file that the wrapper will read from on startup.

[neechbear]

From @iwakeling:

You might like (at some point) to look at the Viper package. It establishes a hierarchy of configuration sources, including default values, config files, command line options, environment variables (I think) and even, if you want it, remote key-value stores like Consul.