ceph-volume: migrate unit tests from 'mock' to 'unittest.mock'

ceph-volume: migrate unit tests from 'mock' to 'unittest.mock'

unit tests in ceph-volume was still using the external 'mock' library
for unit tests, which is unnecessary since 'unittest.mock' is part
of the Python standard library (available since Python 3.3).
This commit updates all imports to use 'unittest.mock' instead,
ensuring better maintainability and removing the need for an extra
dependency.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>