Add environment and handle corner cases

[wangdian]

1. Add dev-box environment for storage plugin.
2. Handle case when setup storage server with no storage user.

[coveralls]

Coverage remained the same at 52.932% when pulling def4a54 on diawang/storage_fix into 06f7311 on master.

[coveralls]

Coverage remained the same at 52.932% when pulling def4a54 on diawang/storage_fix into 06f7311 on master.

[wangdian]

Test case:

1. Use a clean PAI instance or check you k8s configmap in [your ip]/kubernetes-dashboard/#!/configmap?namespace=default, backup storage-user and remove it.
2. In dev box. Go to /pai/contrib/storage_plugin, run
   python storagectl.py server set [teamname] nfs [nfs ip] [nfs root path] --sharedfolders data --privatefolders users
3. The test case passed if no exception occurs.
4. Recover storage-user configmap