Allow cache_git.sh to handle additional projects adhoc
Allow cache_git.sh to handle additional projects adhoc

When testing external plugins it's extremely useful to be able to
cache additional repositories with cache_git.sh. This allows us to
just pass a whole set of additional items on the cli that will also
get cached.

Change-Id: I966c45efa9e8eb8868323de74f363eb1990e534e

openstack-dev/grenade,openstack-dev/grenade,dtroyer/grenade,dtroyer/grenade