Adds VPN Handler

JIRA: N/A

adds a vpn handler in source/api/vpn.py
vpn.py contains a formal interface definition as well as a specific
implementation using LDAP. If your lab's vpn does not use LDAP, you may
create a new vpn handler that properly extends the abstract vpn class
and it should be fine.

Change-Id: I31e8d8477dfed913c4da864d3ff3b49e988d64b1
Signed-off-by: Parker Berberian <pberberian@iol.unh.edu>