tasks/ceph-deploy: allow configurations with no mds

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Change "Exception" to "RuntimeError" as advised by zackc

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #25 from dmick/master

tasks/ceph-deploy: allow configurations with no mds