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.