Removing duplicated classes from lbaas.py and vpn.py

This patch removes duplicated classes AttributeDict from
api/lbaas.py and api/vpn.py. Additional attributes are
added as needed in 'list' and 'get' methods.

Closes-Bug: #1267779
Change-Id: I47d8eb5fc9bd0767ac85a7c65d394ab2b7a2ea5d
Merge "Removing duplicated classes from lbaas.py and vpn.py"