Handle non default realms in rgw user create

[theanalyst]

clusters can have other realms configured as default which wouldn't map to the name 'default', we need to create users for the configured default realm. So in a non multisite cluster;

$ radosgw-admin realm list  # empty 
$ radosgw-admin realm get  # -enoent

for a multiste cluster

$ radosgw-admin realm list # json of default realm id and list of realms
$ radosgw-admin realm get # json of default realm

With the above outputs determine the default realms and use this value if populated

[swiftgist]

The related update would be /srv/salt/ceph/rgw/files/users.j2 where

realm:
  default:
{% if salt['file.file_exists']('/srv/salt/ceph/rgw/users/system.user.yml') %}

would become

realm:
  {{ salt['rgw.default_realm']() }}:
{% if salt['file.file_exists']('/srv/salt/ceph/rgw/users/system.user.yml') %}

and we need to create a default_realm function. I suspect the users function would need to change its default as well.