nfs/rgw: support enforcing keys

nfs/rgw: support enforcing keys

if one sets `ceph_nfs_rgw_access_key` and/or `ceph_nfs_rgw_secret_key`,
the nfs/rgw user creation won't take those variables into account and it
will generate a user with automatically generated credentials.
It ends up with a mismatch between what will be set in ganesha.conf and
the created user.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=2010754
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>