api : Add actions resume support

Add support for resume action using a similar interface to that provided
by nova for the admin actions extension.  So a body of {'resume': None}
will suspend the stack, see http://api.openstack.org/api-ref.html

Change-Id: I89bbafbe2f8b2503f241659a36b83ceab605e763
blueprint: stack-suspend-resume
engine : resume support for nested stacks

Change-Id: I82da46c6f18c035927c6305968470db613d7229c
blueprint: stack-suspend-resume
Merge "api : Add actions resume support"
Merge "engine : resume support for nested stacks"