crush: silence error messages in unit tests

The error messages are intentional when error conditions are
created. They will create false positive in the gitbuilder parser when
the string error is found.

The --debug-crush flag is detected to allow the caller to reset the
verbosity level.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #966 from dachary/ceph-master

crush: silence error messages in unit tests

Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>