BootstrapSuite.TestBootstrapProviderDetect(No)Regions fails the expected auth type due to misordering

[Nicholas Skaggs]

As seen in http://reports.vapour.ws/releases/issue/57c5ccbb749a564e161e47f0;

    BootstrapSuite.TestBootstrapProviderDetectRegions and
    BootstrapSuite.TestBootstrapProviderDetectNoRegions fails the expected auth type due to misordering:

[LOG] 0:00.179 INFO juju.cmd.juju.commands combined bootstrap constraints:
bootstrap_test.go:1113:
    c.Assert(bootstrap.args.Cloud, jc.DeepEquals, cloud.Cloud{
        Type: "dummy",
        AuthTypes: []cloud.AuthType{cloud.EmptyAuthType, cloud.UserPassAuthType},
        Regions: []cloud.Region{{Name: "bruce", Endpoint: "endpoint"}},
    })
... obtained cloud.Cloud = cloud.Cloud{Type:"dummy", AuthTypes:cloud.AuthTypes{"userpass", "empty"}, Endpoint:"", IdentityEndpoint:"", StorageEndpoint:"", Regions:[]cloud.Region{cloud.Region{Name:"bruce", Endpoint:"endpoint", IdentityEndpoint:"", StorageEndpoint:""}}, Config:map[string]interface {}(nil), RegionConfig:cloud.RegionConfig(nil)}
... expected cloud.Cloud = cloud.Cloud{Type:"dummy", AuthTypes:cloud.AuthTypes{"empty", "userpass"}, Endpoint:"", IdentityEndpoint:"", StorageEndpoint:"", Regions:[]cloud.Region{cloud.Region{Name:"bruce", Endpoint:"endpoint", IdentityEndpoint:"", StorageEndpoint:""}}, Config:map[string]interface {}(nil), RegionConfig:cloud.RegionConfig(nil)}
... mismatch at .AuthTypes[0]: unequal; obtained "userpass"; expected "empty"

Similar to last month's https://bugs.launchpad.net/juju/+bug/1604561.

Introduced in https://github.com/juju/juju/commit/f5c7ebb3456fef2e294e0b3c75626cb3bc805a99