ceph-create-keys: add missing argument comma

ceph-create-keys: add missing argument comma

The arguments "get" and "client.admin" were being concatenated into
"getclient.admin".

Found using ceph-ansible + strace:

    13031 execve("/usr/bin/ceph", ["ceph", "--cluster=ceph", "--name=mon.", "--keyring=/var/lib/ceph/mon/ceph-ceph-mon0/keyring", "auth", "getclient.admin"], ["PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin", "LANG=en_US.UTF-8", "CLUSTER=ceph", "TCMALLOC_MAX_TOTAL_THREAD_CACHE_BYTES=134217728", "CEPH_AUTO_RESTART_ON_UPGRADE=no"] <unfinished ...>

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>