ceph-dashboard: remove bool filter for rgw vars

ceph-dashboard: remove bool filter for rgw vars

Some dashboard_rgw_api_* variables are using the bool filter but those
variables are strings with an empty string as default value.
So we should test the variable against an empty string instead of a
bool.

dashboard_rgw_api_host: ''
dashboard_rgw_api_port: ''
dashboard_rgw_api_scheme: ''
dashboard_rgw_api_admin_resource: ''

Resolves: #4179

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>
(cherry picked from commit 5413274412394c314df578ddf5208b79af75f222)