Conform CephExternal template to the new hiera hook

Conform CephExternal template to the new hiera hook

We missed to refactor CephExternal when migrating to the new
hiera hook. The old template would have pushed the value of
ceph::profile::params::client_keys as a string causing the
deployment to fail with:

  Error while evaluating a Function Call, {...} is not a Hash

The new template emits that same data as a map, as it happened
for the other services in Ibe7e2044e200e2c947223286fdf4fd5bcf98c2e1

Change-Id: I3cf59b7d8343d7433047e9ccef310d287dbd47b5