api : Add ReST actions POST method

api : Add ReST actions POST method

Initial support for an actions subpath, which will be the access-point
for non-lifecycle operations on stacks, e.g suspend/resume

This adds support for a  POST method for /actions subpath, so that requests for
specific actions may be made, using a similar interface to that provided by
nova for the admin actions extension.  So a body of {'suspend': None}
will suspend the stack, see http://api.openstack.org/api-ref.html

blueprint: stack-suspend-resume
Change-Id: I180345c818cbfa38ac8c17caba2c0f64adabf6be