Adding the ability to selectively cache only some git repos in
nodepool slave.
This is useful in some environments where it is needless to cache
all the git repos that appear in gerrit.
For example, in a 3rd party CI environment, each nodepool slave
needs only few openstack projects in order to check commits to
a specific project. So it is needless to cache > 1500 projects
currently in gerrit
Change-Id: Ie7302594b24d2bdcc157d3cf64b1f219e7ef0205
918a386596