rbd: manage admin key from a new class

Previously, the key was managed from the site.pp which is not the best
way when moving to Hiera.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
rbd:key: fix class

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
rbd:key: delete empty line

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #443 from enovance/rbd-key

rbd: manage admin key from a new class