Various fixes in the Profitbricks driver:

1. Fix "ex_describe_datacenter" to return a single object instead of a list
2. Fix "ex_describe_datacenter" method signature to take an id instead of an object
3. Fix default value for "state" argument in the "_wait_for_datacenter_state" method
4. Fix while condition in the "_wait_for_datacenter_state" method
5. Modify "_wait_for_datacenter_state" to throw an Exception on timeout
6. Do the same fixes in the "_wait_for_storage_volume_state" method
7. Fix "ex_create_datacenter" to return single object instead of a list
8. Fix "create_volume" to return a single object instead of a list
9. Fix "ex_describe_node" to return a single object instead of a list
10. Fix "create_node" to retirn a single object instead of a list
11. Fix "attach_volume" to return volume on success
12. Fix "ex_create_network_interface" to return a single object instead of a list
Merge pull request #1 from Kami/LIBCLOUD-589_profitbricks_compute_driver

Various fixes in the Profitbricks driver: