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
Merge pull request #276 from axw/lp1339240-mongo-replset-retry-1.20

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.

(Cherry-pick from master)

Fixes https://bugs.launchpad.net/juju-core/+bug/1339240