ceph::key : add keyring and id key injection arguments

The id and keyring used when injecting a key do not necessarily match
the ceph default. The inject_as_id and inject_keyring arguments are
added to change them independently of each other.

The argument documentation is updated and the integration tests upgraded
to demonstrated the intended side effect.

Change-Id: I9b19a66558bc11e16da9fde4f11d23b850154186
Signed-off-by: Loic Dachary <loic@dachary.org>