provider/azure: tests are racy

[Roger Peppe]

go test -race launchpad.net/juju-core/provider/azure

prints many races.
I was prompted to look at this from an intermittent test failure
on my machine:

 FAIL: environ_test.go:766: environSuite.TestStopInstancesWithLimitedConcurrency

 environ_test.go:783:
     c.Check(err, gc.IsNil)
 ... value parallel.Errors = parallel.Errors{(*errors.errorString)(0xc21000a090)} ("EOF")

 environ_test.go:784:
     c.Check(len(*requests), gc.Equals, len(services)*2)
 ... obtained int = 19
 ... expected int = 20

 OOPS: 158 passed, 1 FAILED
 --- FAIL: TestAzureProvider (7.45 seconds)
 FAIL
 FAIL launchpad.net/juju-core/provider/azure 7.788s

The full output from go test -race is rather long, but I've attached the output from
running "go test -race -gocheck.f TestStopInstancesWithLimitedConcurrency"

[Andrew Wilkins (axwalk)]

This was fixed at the same time availability sets were implemented.