general _get_instances via pool info
cluster inherits from RPC
Template has distinct key name for pool info
simplify code with _get_instances method
add Datastore class
add get_datastores method
determine datastore name
use data, not extended_data
use correct class for get_vms and get_templates
Merge pull request #6 from stdweird/datastore

refactor the get_ methods, and add Datastore support