Add an API to converge a stack or resource

Add new API or options to existing APIs to invoke converge on resource.

For example, CLI would look like:

heat stack-update --converge <stack_name>

heat stack-update --converge <stack_name> <resource_name>

Equivalent restful APIs need to be added.

This will help user manually trigger convergence on resources or stack. A user could do a stack-check to see changes in state and he/she should be able to invoke convergence on resource or whole stack.

There will be continuous observer in future to do this automatically. But this will still be desired for user who don't want to continuously monitor the stack/resource but would like to have a way to trigger converge manually.