Specify a smaller mongo oplog size on local provider

Since the local provider has a single Mongo instance,
the oplog is unuseful. We reduce the size to speed up
preallocation.
Merge pull request #274 from axw/lp1338179-local-oplog-1.20

Specify a smaller mongo oplog size on local provider

Since the local provider has a single Mongo instance,
the oplog is unuseful. We reduce the size to speed up
preallocation.

(Cherry-pick from master)

Fixes https://bugs.launchpad.net/juju-core/+bug/1338179