Allow to specify IP address of floating ip

IP address of floating ip will be automatically allocated.
There are cases where users need to specify a floating IP address.
This patch addresses the problem.

The feature is limited by "create_floatingip:floating_ip_address" in
"policy.json".
By default, it needs an admin role.

DocImpact
APIImpact

Implements: blueprint allow-specific-floating-ip-address

Change-Id: Iba64a0f0a38ca5eb39c605e121a12c956637b96c
Merge "Allow to specify IP address of floating ip"