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.
Typically, for each method that return an error (almost all methods should) there is a public `XyzErr error` field so that the error can be set to allow testing negative paths.