Select resources to migrate by ids/tenant id

Add 2 optional mutually exclusive arguments:
* list of ids of resources to migrate (--id)
* tenant id of resources to migrate (--tenant)

Former is supported for all resource types.
Latter is supported only for resources that can be grouped by
tenant id.

Add function for filtering the list of resources by tenant id.
Add placeholder function to get the list of all resources
of the specified type.
Ohhh, of course the example show how to check that the args.ids is not empty! The example does what you want. I'm sorry for the typo.