ML2 BigSwitch: Don't modify parent context

Makes a copy of the port context before changing
it in preparation for the backend controller so
other drivers are not affected. Also removes a
UT that was exercising direct modification of VIF
details in the port context, which is not allowed
by the ML2 plugin.

Closes-Bug: #1294308
Change-Id: I47281dcd23c022813b8b6eda0a3d39c4482277b9
Merge "ML2 BigSwitch: Don't modify parent context"