Kernels

add kernel management support
This cannot work as datacenter will only be a string, thus it doesn't have a usable_id method.  You could call `gandi.datacenter.usable_id(datacenter)` to fix this to work.

As a general guideline, I prefer to keep internal stuff/checks in modules and be lightweight in commands. For this reason usable_id methods are always kept private in modules and not exposed in commands. 

So to resolve this, I suggest you update Datacenter.list method in datacenter module to accept a datacenter and a vm parameters (default to None) and made the check and selection of proper dc_id filter there. You can check Image.list or Disk.list_create for something similar.