Adds option to select a different subnet when adding 'vip'

With this fix, 'AddVip' from the dashboard gives user an
option to select a different subnet than the 'pool' subnet
and then creates a 'vip' with an ip (provided/next available)
from the selected subnet.

Change-Id: I25f36090c5fb739256b62a88915f6414e39a915f
Closes-Bug: #1285504
Merge "Adds option to select a different subnet when adding 'vip'"