Reuse the webadmin server and protocol in UI plugin

This makes sure we won't have http/https conflicts and use
the proper endpoints to talk to REST API in UI plugin.

Change-Id: I258a46e3ea9951ffa473572bec6163e3614c9f45
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1140721
Signed-off-by: Martin Sivak <msivak@redhat.com>