ceph_volume: Adding Device.is_valid()

ceph_volume: Adding Device.is_valid()

A block device can be filtered-out/ignored because it have features that
doesn't match Ceph's expectations.

As of today, the current code was rejected removable devices but it was
pretty hidden from the user, and implicit in the get_devices() function.

This patch is creating a new is_valid() function to perform all the
rejection tests and returns if this device can be used in the Ceph
context or not.

If is_valid() is returning False, the 'rejected_reasons' list reports all
the reasons why that devices got rejected.

Signed-off-by: Erwan Velu <erwan@redhat.com>