Floating ip assignement support

'floating_ip_pool' field added to Node Groups
and Node Group Templates.

'floating_ip_pool' may be Network ID or Floation IP pool name.

If this field is not None then Savanna will:
* create a Floating IP for each instance in the Node Group
* assign it on Cluster start or scaling
* remove it on Cluster Deletion.

Validation added for 'floating_ip_pool' field.

Docs and Sample Configs update

Co-Authored-By: Andrew Lazarev <alazarev@mirantis.com>

Fixes bug: #1186874
Implements blueprint ips-assignment-mechanism

Change-Id: Id7438b533d140916d683e7ca47424aa00b66a1a0
Merge "Floating ip assignement support"