auth: 'ceph auth import -i' overwrites caps, if caps are not specified

auth: 'ceph auth import -i' overwrites caps, if caps are not specified
in given keyring file, should alert user and should not allow this import.
Because in 'ceph auth list' we keep all the keyrings with caps and importing
'client.admin' user keyring without caps locks the cluster with error[1]
because admin keyring caps are missing in 'ceph auth'.

[1] Error connecting to cluster: PermissionDeniedError

Fixes: http://tracker.ceph.com/issues/18932
Signed-off-by: Vikhyat Umrao <vumrao@redhat.com>