EVCs should only be pushed to switches when they're not in the flow table yet

[ajoaoff]

Original issue opened by @viniarck at kytos#32.

Hi,

I was talking to Jeronimo about the blueprint, and I realized this requirement wasn't explicitly mentioned:

• EVCs should only be pushed to switches when they're not in the flow table yet.

The reason why this is critical is that if you overwrite an existing flow entry which has network traffic, there will be a short traffic outage since the rule will be installed in hardware. So, mef_eline should have a logic to push only the diffs (inexisting EVCs in the switch), except for when the user request a forced reprovision, which in this case, it doesn't matter anymore.