Use ceph git repo instead of github.

This code change is so that instead of pulling the tarball of github
which can be unreliable at times it instead uses the ceph repo mirror
and serves as the same function. Now it is using git archive and no
longer uses wget. Because of this less tar-fu is needed to extract
the necessary files as it can be done directly through git archive.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>
Reviewed-by: Sam Lang <sam.lang@inktank.com>
(cherry picked from commit 5d66c9ab01814388f3439690031d9875e89ef892)