Admin dashboard shows too many panels now.
This commit introduces panel groups for Admin dashboard
as we do for Project dashboard.
The new order of admin panel groups are:
Compute, Volume, Network and System.
Note that the slug of "System" panel group is not changed
to avoid breaking horizon plugins which register panels to
Admin System panel group.
Implements blueprint reorganize-admin-dashboard
Change-Id: I533e60f7cbfab2539b9f8aa7044f7234165f2518