Finish renaming of cinder_gluster_servers to cinder_gluster_peers

Commit 9bdf1ddc31b28c47e6dc41530a9c8610d8c54555 started a rename of
cinder_gluster_servers to cinder_gluster_peers. This completes that change
with the rename of this parameter for the standalone hostgroups.
Merge pull request #226 from bcrochet/gluster-peers

Finish renaming of cinder_gluster_servers to cinder_gluster_peers