rgw: refact rgw pools creation

rgw: refact rgw pools creation

Refact of 8704144e3157aa253fb7563fe701d9d434bf2f3e
There is no need to have duplicated tasks for this. The rgw pools
creation should be delegated on a monitor node se we don't have to care
if the admin keyring is present on rgw node.
By the way, only one task is needed to create the pools, we just need to
use the `docker_exec_cmd` fact already defined in `ceph-defaults` to
achieve it.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1550281
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>