TestAddServiceStorageConstraints fails on ppc64

A new test introduced as part of the storage pools work is consistently failing in the ppc64 job.

<http://reviews.vapour.ws/r/920/>
<http://reports.vapour.ws/releases/2334>

This may be gccgo related rather than ppc64, for instance a map-ordering issue.

<http://data.vapour.ws/juju-ci/products/version-2334/run-unit-tests-trusty-ppc64el/build-2351/consoleText>

----------------------------------------------------------------------
FAIL: storage_test.go:58: TestAddServiceStorageConstraints.pN49_github_com_juju_juju_state_test.StorageStateSuite

SetUpTest
...
SetUpTest done
storage_test.go:65:
    c.Assert(err, gc.ErrorMatches, expect)
... error string = "cannot add service \"storage-block2\": charm \"storage-block2\" store \"multi2up\": 2 instances required, 1 specified"
... regex string = "cannot add service \"storage-block2\": no storage pool specified and no default available .*"

[LOG] 0:00.082 INFO juju.testing reset successfully reset admin password
OOPS: 787 passed, 1 FAILED
--- FAIL: TestPackage (249.44 seconds)
FAIL
FAIL github.com/juju/juju/state 250.149s