clear fallback interface before reprovisioning

[anhvoms]

Proposed Commit Message

Azure: fallback nic needs to be reevaluated during reprovisioning

During reprovisioning, VM network will change. fallback nic
should be cleared after use so that it can be re-evaluated after
reprovisioning

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly