Allow choice in how SignalResponder user id is stored.

Some subclasses of SignalResponder may assign a resource_id
with the real reasource that it creates.

With this change, the user_id is always stored in resource data.
If user_id resource data is missing an assumption is made that
this resource was created before this commit, so the
resource_id is used as the user_id.

Subclasses of SignalResponder which previously used the
user ID as their resource_id now need to call resource_id_set
themselves during handle_create.

Change-Id: Iac41ab99f8e9b0953b2cd447ff0e027443fabeca
Fix syntax error in docs

There are several syntax errors in docs.
The patch will add missing punctuation, remove extra whitespace, and
change the "heat-api-engine" to "heat-engine" in heat-engine.rst.

Change-Id: I67dcad5e0e0a1f71a67ab8fc4311d96db4820c29
Fix OS::Neutron::Pool validation

Validation fails for resource OS::Neutron::Pool
if session_persistence property of VIP is set to None

Change-Id: Idb837c7660c8e140c4ff81ead41bb38c75577215
Closes-bug: #1270740
Add a new ThreadGroupManager class

Introduce a ThreadGroupManager class to manage active ThreadGroups for
engine services and move the thread-related EngineService methods to
ThreadGroupManager.  This will make it easier for multiple engine
services to manage thread groups.

Partial-Bug: #1262012
Change-Id: I1ce7785021726c274a294ef402b036540d395a29
Merge "Fix OS::Neutron::Pool validation"
Merge "Allow choice in how SignalResponder user id is stored."
Merge "Add a new ThreadGroupManager class"
Merge "Fix syntax error in docs"