Set proper SSH URL for all GIT repos (with username) - take 2

git_user can now be:
* empty -- git remotes are not managed by vagrant
* 'anonymous' -- git sets remotes to 'origin' -> https
* <your user name> -- git sets remotes to 'gerrit' -> ssh

If you previously entered empty value when asked, your remotes
will not be updated unless you edit .settings.yaml file.

Change-Id: Id5938b55093afde571dd67e4e93d5ca98379d5f3