Extend Network Service policy to represent the user intent to have
external connectivity for all PTs in a PTG.
The floating IP is allocated from the nat_pool associated with the
external segment associated to the PTGs l3 policy
Change-Id: I58ea3afb374ab61f57f9a40eab0acd4750133ec7
Implements: blueprint gbp-floating-ip-support
6cf6b03125