allow stackdriver host project to differ from monitored project

As noted in #11, 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