iaas/cloudstack: fix race condition by waiting for queue to finish
iaas/cloudstack: fix race condition by waiting for queue to finish

Example of a race caught by the race detector:

```
==================
WARNING: DATA RACE
Read at 0x00c42021dd40 by goroutine 67:
  github.com/tsuru/tsuru/iaas/cloudstack.(*cloudstackSuite).TestCreateMachineAsyncFailure()
      /home/travis/gopath/src/github.com/tsuru/tsuru/iaas/cloudstack/iaas_test.go:140 +0x5dc
  runtime.call32()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/runtime/asm_amd64.s:509 +0x3a
  reflect.Value.Call()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/reflect/value.go:302 +0xc0
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.(*suiteRunner).forkTest.func1()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/check.go:772 +0x9ab
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.(*suiteRunner).forkCall.func1()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/check.go:666 +0x89
Previous write at 0x00c42021dd40 by goroutine 17:
  github.com/tsuru/tsuru/iaas/cloudstack.(*cloudstackSuite).TestCreateMachineAsyncFailure.func1()
      /home/travis/gopath/src/github.com/tsuru/tsuru/iaas/cloudstack/iaas_test.go:117 +0x157
  net/http.HandlerFunc.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:1918 +0x51
  net/http.serverHandler.ServeHTTP()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2619 +0xbc
  net/http.(*conn).serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:1801 +0x83b
Goroutine 67 (running) created at:
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.(*suiteRunner).forkCall()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/check.go:663 +0x430
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.(*suiteRunner).forkTest()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/check.go:754 +0x131
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.(*suiteRunner).runTest()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/check.go:809 +0x42
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.(*suiteRunner).run()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/check.go:615 +0x1fa
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.Run()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/run.go:92 +0x5a
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.RunAll()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/run.go:84 +0x136
  github.com/tsuru/tsuru/vendor/gopkg.in/check%2ev1.TestingT()
      /home/travis/gopath/src/github.com/tsuru/tsuru/vendor/gopkg.in/check.v1/run.go:72 +0x680
  github.com/tsuru/tsuru/iaas/cloudstack.Test()
      /home/travis/gopath/src/github.com/tsuru/tsuru/iaas/cloudstack/iaas_test.go:21 +0x38
  testing.tRunner()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/testing/testing.go:746 +0x16c
Goroutine 17 (running) created at:
  net/http.(*Server).Serve()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/server.go:2720 +0x37c
  net/http/httptest.(*Server).goServe.func1()
      /home/travis/.gimme/versions/go1.9.2.linux.amd64/src/net/http/httptest/server.go:280 +0xa2
==================
```

guilhermebr/tsuru,tsuru/tsuru,guilhermebr/tsuru,guilhermebr/tsuru,ggarnier/tsuru,ggarnier/tsuru,guilhermebr/tsuru,tsuru/tsuru,tsuru/tsuru,ggarnier/tsuru,tsuru/tsuru,ggarnier/tsuru