use pool of repositories to clone wiki repos

After this change, the chart can use a pool of prepopulated wiki repos present at the location pointed by the chart value reposCache.wikiRepos. This pool will be used to save disk space and reduce network usage, the latter also speeding up environment creation.

The pool is initially created by Patchdemo, which at the moment is the main client of this chart. So the directory structure of the pool is expected to be the one Patchdemo creates

Bug: T375236