StackUser add suspend/resume support

Adds suspend resume support which will be required to migrate the
User resource to this base class without losing functionality

Change-Id: I3607ef71bfcd863b08879b3856baa92e7aeeef10
blueprint: instance-users
Add test for StackUser._create_keypair

Currently _create_keypair is only indirectly tested via SignalResponder
so add a base-class-specific test.

Change-Id: Ic9f1df06df25205f864b2f0138d2821089608b88
blueprint: instance-users
Merge "StackUser add suspend/resume support"
Merge "Add test for StackUser._create_keypair"