Need regression test to verify switch dpid assignment

[mininet]

Need regression test to verify switch dpid assignment

[moijes12]

Tests can be written for the below cases

1. Default dpid is assigned if no dpid is not passed in switch creation
2. Switch dpid is Actual dpid is assigned if dpid is passed in switch creation
3. Default dpid assignment fails if the name of the switch does not contain a digit. The Exception message is 'Unable to derive default datapath ID - please either specify a dpid or use a canonical switch name such as s23.'
4. Default dpid is lenght is 16 characters consisiting of 16 - len(hex of first string of contiguous digits passed in switch name) 0's followed by hex of first string of contiguous digits passed in switch name.

@lantz Do we need to add more ?

[moijes12]

Submitted regression tests in pull request #235

[moijes12]

Please use pull request #241

[cheriimoya]

can this be closed? or are there more tests needed?