Make OS::Nova::Server networks property updatable

This patch adds the ability to update the networks property.
Update process consists of two simple steps:
    1. detaching all old interfaces
    2. attaching all new interfaces.

Change-Id: Iaabf8be1adc6cb7d5ecc08edb92d9f9ce029b125
Closes-bug: #1250645
Merge "Make OS::Nova::Server networks property updatable"