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

Users will be prompted to enter git username if .settings.yaml
does not exist.

If .settings.yaml specifies git user (e.g. GIT_USER: "yurik" ),
the URL becomes proper SSH-based URL *after* the git is cloned.
Also the remote name is changed from "origin" to "gerrit".

Change-Id: I04cdee7b83e2b1c3503f4b73c902d08ec7028904
Merge "Set proper SSH URL for all GIT repos (with username)"