This patch provides the ability for the repo server to take advantage
of a pre-staged git cache on the deployment host, such as that provided
on OpenStack-CI's nodes.
This reduces the execution time of the build process by ensuring that
the git clone process only has to complete an update, instead of a
fresh clone.
The patch includes a change to the AIO bootstrap which checks for the
existance of a git cache in the location it is typically found in
OpenStack-CI. If that path exists it sets the repo_build_git_cache
variable. In a case where the AIO is not being built in OpenStack-CI
then the deployer may either use the same path, or provide the path
as an extra option when bootstrapping.
Change-Id: I0c9d574a7f694cd3772a68fcdc71673616ea8e3b