Possible EVC duplication for UNIs with no TAGs

[Alopalao]

mef_eline checks for EVC duplicates when a UNI has not TAG in the method _check_no_tag_duplication() from main.py. This method is not thread-safe and it is placed before adding the EVC to self.circuits. Two EVCs with UNIs without TAGs created simultaneously is theoretically possible.

Possible Solution:

Add "none" to default_special_tags property from Interface.

[viniarck]

@Alopalao, good finding, yes self._check_no_tag_duplication needs to have a mef_eline global threading.Lock. That should suffice.

The "none" option on special goes against what we consider special, special "any", "untagged" they all have OpenFlow semantics, while "none" would be confusing with EPL.