webadmin: Set Provider system tree icon by type

Added ability to set an icon for a Provider in the system tree specific
to the type of the provider, at the moment icons exist for OpenStack
Network and Foreman.

Change-Id: I9f689bc4d9ab1227da8f1e763bf8149ec045a8ce
Signed-off-by: Lior Vernia <lvernia@redhat.com>