Add a new Settings.gitlab value "user_home".

Instead of assuming that the Settings.gitlab['user'] home directory is:
/home/git

Obtain the default value from Etc and use that if not set in config.yml.
Apparently the gitlab Travis install has no git user.

Hackily handle this with a begin block and return a default that matches current use.
Merge pull request #4037 from mitchty/master

Remove user directory path assumptions