ceph: set external creds if admin key is available
ceph: set external creds if admin key is available

If the admin key is available, set the external creds struct
to use that admin username and key instead of requiring the
callers to check every time if they should use the admin
key or the external key.

Signed-off-by: Travis Nielsen <a8b474fdb3f77f44446f0ab9ddca8147b09b3a9e@redhat.com>

rook/nfs,rook/rook,rook/nfs,rook/rook,rook/rook,rook/nfs,rook/rook,rook/rook