Delete models.py and clean get_members()

The Member object is not very useful since we interact with
members only by ids and capabilities.

Currently get_members() retrieves the list of member_ids and then
retrieves the capabilities for each members in order to return a
list of Member object. This will actually complicate the asynchronous
API because we will need to wrap several Kazoo asynchronous calls
into one Tooz asynchronous call.

So now get_members() will simply return the list of members ids,
capabilities can be retrieved by using get_member_capabilities().

Change-Id: I407042df8adc274fc48f7a4e58b7503a732754a0