network: set the default floating ip pool name

The default public network used by packstack to create
floating ip is 'public' we should configure nova.conf to use
the good value during the build process to provide the possibility
to use the command 'floating-ip-create' without the optional
argument 'pool'.

Closes-Bug: #1356462
Related-Bug: #1355621
Change-Id: Ie5158b44dee320816fa4eae20bb2e28a4e58c451
Merge "network: set the default floating ip pool name"