Support /etc/ssh/ssh_config

[flosell]

We rely on JGit and JSch to handle Git over SSH for us. Their implementation however is a bit messy:

• JGit reads ~/.ssh/config but not `/etc/ssh/ssh_config
• We would expect that both files are read to mimic OpenSSHs behavior.
• JSch also implements parsing but is probably overwritten by JGit

Investigate what we can do to improve this, maybe through a pull request or by at making the config file to be used configurable.

Related issues:

• Allow customizing StrictHostKeyChecking #22
• JGit change 990670 would introduce better ssh-config handling (but still only ~/.ssh/config)

[arichiardi]

Very weird that only one is read.