REST deployment metadata method

A GET on the path /software_deployments/metadata/<server_id>
will return a list of software configs which should be deployed
to that server.

See the previous commit for the justification for this method.

In the future servers will be able to poll for this directly so
the policy.json does not have the deny_stack_user rule. Now that
this method exists software_deployments:index can now enforce
deny_stack_user.

partial blueprint hot-software-config-rest

Change-Id: I514677850cfb742bec85a5db75c71e6b2c7c9ea2
SignalResponder move signed URL deleting to its own method

Subclasses may not be handling this in their handle_delete
methods

Change-Id: I3dbb89e0a86690617613b25c9533bbf52d1da8a9
Merge "REST deployment metadata method"
Merge "SignalResponder move signed URL deleting to its own method"