Added API doc for VPNaaS, based on the initial code

Used the data model information from the Wiki page at
https://wiki.openstack.org/wiki/Neutron/VPNaaS, cleaned up some
of the syntax, invoked the commands with --verbose option,
and collected JSON requests and responses. Spell checked.

bug 1203865

Change-Id: Iae57319a0fab61d4f0a82f2ea168d6a2c841e9ca
Merge "Added API doc for VPNaaS, based on the initial code"