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.
Fix check for empty list
Implement 'all' migration strategy

Function get_all_resource_ids returns a list of IDs of all resources for
the given type of resources.
[doc] Add docstring to functions defining migration strategies
Remove obsolete TODO line
Add function to group by host ('host' migration strategy)
Add argument for filtering servers by host
Handle the 'host' argument in main() function
Merge branch 'master' into feature/migration-strategies
Typo fixed
[doc] Add return value to docstring
Merge pull request #5 from MirantisLabs/feature/migration-strategies

Feature/migration strategies