Reopen mongo sessions between replset initiation attempts

Also, reinstate a change made by mfoord to call
CurrentStatus after attempting intiation, and
refresh session between attempts.

https://bugs.launchpad.net/juju-core/+bug/1339240
Extend replSetGetStatus timeout, add a test
Merge pull request #271 from axw/lp1339240-mongo-replset-retry

Reopen mongo sessions between replset initiation attempts

Also, reinstate a change made by mfoord to call
CurrentStatus after attempting intiation, and
refresh session between attempts.

https://bugs.launchpad.net/juju-core/+bug/1339240