Test the namespaces listing endpoint

[LucasRoesler]

Each provider that supports multiple namespaces needs to support list available namespaces, the endpoint is /system/namespaces. This should be the first test case we add for multinamespace support as part of #61

https://github.com/openfaas/faas-provider/blob/bc0f26cae70c1b3ba2a595cea8bfb8c04925d5c6/serve.go#L68

For faas-netes the implementation is here https://github.com/openfaas/faas-netes/blob/e5cf64dcf4d01f04636d2ffe736be110981b83c5/pkg/handlers/namespaces.go#L22-L48 It requires deployment with a ClusterRole and annotations added to the permitted namespaces.

We need to be able to confirm both of these cases:

1. when deployed with multi-namespace support disabled, the endpoint should only return a single namespace, the default namesapce (usually openfaas-fn)
2. when deployed with multi-namespace support enabled, it should return it should return each of the configured namespaces and it should also include the default namespace.

I don't think it requires writing multiple test cases, rather the ideal situation is that the one test can work to run against deployments/configurations.

[nitishkumar71]

I will work on this to as next step to support #61