Reorganize Admin Dashboard menu

[Motivation]
All the panels are listed directly under the Admin dashboard currently.
We have now a number of panels and it is better to have some grouping from the perspective of user navigation.

[Proposal]
Introduce the following panel groups into the Admin dashboard.

- (Directly under Admin Dashboard) Overview
- Compute : Hypervisors, Host Aggregate, Instances, Flavors, Images
- Volume : Volumes, (Volume) Snapshots, Volume Types
- Network : Networks, Routers, Floating IPs
- System : Defaults, Metadata definitions, System Information

The proposed order of panels are as above. It is the same order as for Project dashboard.

[Impacts on Horizon plugins]
Horizon plugins which register panels to the Admin dashboard need to prepare panel groups if necessary and update panel configuration appropriately. Until then, all panels registered to "System" panel group previously continues to be displayed in "System" panel group.

[Upgrade Impact]
Operators who customize panel group configurations need to update their configurations accordingly. For example, if an operator disables some panels in Admin dashboard through 'enabled' directory, the panel group in 'enabled' file needs to be updated.

[Detail]
Proposed numbering of enabled files:
_20xx : top level under Admin dashboard
_21xx : compute panel group
_22xx : volume panel group
_23xx : network panel group
_28xx : system panel group

Note : _24xx - _27xx : reserved for horizon plugins. They may want to insert panel group before "System" panel group.
Note : _29xx is reserved from the same reason