virStorageBackendRBDOpenRADOSConn: Don't log the RBD key

virStorageBackendRBDOpenRADOSConn: Don't log the RBD key

'virStorageBackendRBDRADOSConfSet' logs its arguments but it's also
used to set the RBD secret/key.

All the security theatre with securely erasing the string we do to fetch
the secret would be quite pointless if we log it thus introduce
virStorageBackendRBDRADOSConfSetQuiet and use it to avoid logging the
password.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>