Fix interface-attach removes existing interfaces from db

The following commit 394c693e359ed4f19cc2f7d975b1f9ee5500b7f6 changed
allocate_port_for_instance() to only return the ports that were created
rather than all of the ports on the instance which broke the attach-interface
code.

This patch fixes this issue by remove the sync decorators from:
allocate_for_instance, allocate_port_for_instance, and
deallocate_port_for_instance to just _build_instance_nw_info which
is called in all these cases instead.

Closes-bug: #1223859

Change-Id: I66eb0c0ab926e0a8d1e2c9cfe1f7fd579ea3aa27
Merge "Fix interface-attach removes existing interfaces from db"