Add API resources for Plans and Plan

Plans are used as a template for creating assemblies. This patch adds a
place to hang documentation for Plan and Plans resources.

Also see: https://etherpad.openstack.org/p/solum-demystified

blueprint api
Change-Id: I56240f79ddbd49d69b9e62b7b2030863eeadf3c5
Co-Authored-By: Angus Salkeld <angus.salkeld@rackspace.com>
Merge "Add API resources for Plans and Plan"