cmdSubnetSuite teardown fails: unexpected message

[Aaron Bentley]

cmdSubnetSuite teardown can fail with: local error: unexpected message: github.com/juju/testing/mgo.go:632: inst.Dial() failed

As seen here:
http://reports.vapour.ws/releases/issue/57b346c1749a566e453861de

FAIL: <autogenerated>:522: cmdSubnetSuite.TearDownTest

<autogenerated>:522:
    ...open <autogenerated>: no such file or directory
/home/ubuntu/juju-core_2.0-beta16/src/github.com/juju/juju/provider/dummy/environs.go:313:
    c.Assert(err, jc.ErrorIsNil)
... value *errors.Err = &errors.Err{message:"inst.Dial() failed", cause:(*net.OpError)(0xc820c02c30), previous:(*net.OpError)(0xc820c02c30), file:"github.com/juju/testing/mgo.go", line:632} ("inst.Dial() failed: local error: unexpected message")
... error stack:
 local error: unexpected message
 github.com/juju/testing/mgo.go:632: inst.Dial() failed