Fix _validate_mac_address method

The method _validate_mac_address previously used: netaddr.EUI(data)
which would convert data passed in to the correct mac_address representation.
For example if data=123 it would return: EUI('00-00-00-00-00-7B'). To fix
this issue I used netaddr.valid_mac() instead which returns a bool if a mac is
valid or not and does not raise. Note this file needs some improvements to
improve how exception handling is done that will come later.

Change-Id: I4fb868e40abbad9e743028cc768f47ca9b3e0e70
Closes-bug: 1291163
Fix wrong section name "security_group" in sample config files

The section name [security_group] in Sample config files of
ML2 and Big Switch plugins was wrong and it should be
[securitygroup].

Change-Id: I8204b2f37f96a5e46038e117853ac8637a9705fd
Closes-Bug: #1304105
Merge "Fix _validate_mac_address method"