PatchPanel refactoring to follow the pattern set for other managers

PatchPanelBK is introduced to help extending the PatchPanel by implementing
the basic book keeping functions that are not really dependent on the testing
infrastructure.
Add simple propperty in HostsManager to list all hosts.
Add a property to SwitchesManager to list all switches
Add helper methods to TopologyGraph
- Retrieve all links connected to a host
- Retrieve all links connected to a switch
Forgot to add the property to the parent class
Add a property to SwitchesManager to list all switches
Rename Mininet Entities to TestON entities

This is more acurate names, since we rely on the TestON drivers.
Remove unnecessary print statement
Change TestONOVSSwitch port to dict

This will make it match the STS switch interface