allow stackdriver host project to differ from monitored project

As noted in #11 (closed), we do not support the common case where a single Stackdriver workspace hosts multiple projects. Do this by requiring a Google provider for the Stackdriver workspace be passed to the module is alerting is enabled.

We use a separate provider because the provider used must be able to create monitoring resources in the host project. In the case of our standard deployment, this implies it needs product admin credentials.

We no longer need to enable the monitoring service since this service a) needs to be enabled in the host project and b) the service will have been enabled as a necessary side-effect of creating the Stackdriver workspace.

Closes #11 (closed)