BGP service: Construct function name with format
BGP service: Construct function name with format

In the BGP service, function names were constructed dynamically using
concatenation. Doing so with string format is more clear.

Change-Id: Iec0f3e50e0ff00ad2b5239325549ed28737dec6c

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow