Update Compute to use provider::service syntax

[sethboyles]

When upgrading to fog-core 2.1, we see a lot of errors like this:

                NameError:
                  uninitialized constant Fog::Aliyun::Compute
                  Did you mean?  Fog::Compute

Updating the syntax should resolve this and make fog-aliyun compatible with fog-core 2.1