Update/add validate functions

This patchset adds the following:

* A `validate_ip_action` for actions in IPs. Currently, it only checks if an IP can be deleted. Previously, this check was embedded in the `delete_floating_ip` action, but the admin interface needs a way to import it.
* A check in `validate_project_action` for suspended projects. With this check, a suspended project cannot get suspended again.

Also, this patchset adds the necessary tests for the `validate_ip_action` function.