Prefix labels of monitoring things with the region.

Because Cloud Monitoring is regionless, running `script/deploy --tags monitoring` in multiple regions will actually result in *only* monitoring the last region that's executed, since `script/rmmon` nukes all existing monitoring checks and all of the labels collide.

Who even knows what does when you run in in two regions at once.

Whoops.

![failure](http://media.giphy.com/media/C4VDtgpxoGK88/giphy.gif)

To put this right:

- [x] Prefix labels with the active region to namespace them. Checks don't need a prefix because they're scoped to a specific entity.
- [x] Change `script/rmmon` to *only* delete things prefixed with the current region.