Adding some improvements to ceph-flocker-driver

[PiotrProkop]

Following installation guide I found out that DESCRIPTION.rst is missing in main directory. Changing required package from rados to python-cephlibs allows to install rados.py using pip. Making test_case default value to None makes it not necessary to include test_case in cluster.yml flocker configuration.

[tomprince]

Theses changes look reasonable, and appear to be necessary to actually test the code.

[robhaswell]

Thanks Piotr!