Add support for disk creation

In this patch, we tried to add the following things:

1. The support of persistent disk creation using softlayer-go client

2. Change the name of fs_disk things to softlayer_disk things

3. Add a fake client for mocking softlayer-go client instead of using existing mock client defined in softlayer-go project. We think it's a better solution for the unit testings, as softlayer-go is also a go lib that cpi project relies on. This new mock will remove the dependency of softlayer-go during unit testing.
same as before, please make sure to add public fields for each method's return value and error, so, e.g., for the method: `CreateIscsiVolume` I would have two public fields:
1. `CreateIscsiVolume_Softlayer_Network_Storage`
2. `CreateIscsiVolumeErr`

And so on.