Removes pre_live_migration need for Fixed IPs

This commit removes the validation that requires Fixed IP Addresses
for pre_live_migration.  The original comment regarding the
pre_live_migration requiring Fixed IP Addresses alluded to uncertainty
about it not being required.  Workloads should not require a Fixed IP
Address for migration, typically just the storage is required to be
similar.

The commit also updates the associated test case to ensure that no
exception is thrown in the pre_live_migration if a fixed IP Address
is not specified.

Change-Id: I2c4d4dae2cb0f00c67f0f731b5080196564b37da
Closes-Bug: #1226226
Merge "Removes pre_live_migration need for Fixed IPs"