ceph-facts: Fix system_secret_key variable handling

ceph-facts: Fix system_secret_key variable handling

This commit fixes the system_secret_key variable not substitued by the
right value and always using the 'system_secret_key' string instead.

$ egrep 'system_(access|secret)_key' group_vars/all.yml
system_access_key: foofoofoofoofoofoofo
system_secret_key: barbarbarbarbarbarbarbarbarbarbarbarbarb

$ ansible-playbook -vv -i hosts site.yml.sample -e rgw_multisite=true
(...)
  - hostname: storage0
    endpoint: http://192.168.100.42:8080
    instance_name: rgw0
    radosgw_address: 192.168.50.3
    radosgw_frontend_port: 8085
    rgw_realm: canada
    rgw_zone: montreal
    rgw_zone_user: justin.trudeau
    rgw_zone_user_display_name: Justin Trudeau
    rgw_zonegroup: quebec
    system_access_key: foofoofoofoofoofoofo
    system_secret_key: system_secret_key

Fixes https://github.com/ceph/ceph-ansible/issues/5150

Signed-off-by: petruha <5363545+p37ruh4@users.noreply.github.com>
(cherry picked from commit 73b3fadb0ed4cd9eba62669c189fa76178431f07)