agent/mongo: preallocate oplog data files

Preallocate oplog data files. If we don't
do this, then mongo is not happy with us
when running on big/slow disks. Probably
because we pass --nopreallocate to mongod.

Fixes https://bugs.launchpad.net/juju-core/+bug/1327940
Merge pull request #82 from axw/lp1327940-prealloc-oplog

mongo: preallocate oplog data files

Preallocate oplog data files. If we don't do this, then mongo is not happy with us when running on big/slow disks. Probably because we pass --nopreallocate to mongod.

Fixes https://bugs.launchpad.net/juju-core/+bug/1327940