mongo: reduce min/max oplog size

Reduce oplog minimum size to 512MB,
and maximum size to 1024MB.

See comments on bug for backstory
and analysis.

Fixes https://bugs.launchpad.net/juju-core/+bug/1344940
Merge pull request #374 from axw/lp1344940-oplog-smaller-1.20

mongo: reduce min/max oplog size

Reduce oplog minimum size to 512MB,
and maximum size to 1024MB.

See comments on bug for backstory
and analysis.

Fixes https://bugs.launchpad.net/juju-core/+bug/1344940