More robust HA upgrade in machine agent

If upgrading mongo to HA only partially
completes such that the replicaset is not
initiated, then we would previously failed
on the second attempt. Make sure we use
"Direct" connections when upgrading to HA.

Possibly fixes https://launchpad.net/bugs/1334273
Merge pull request #187 from axw/machine-agent-upgrade-ha-robustness

More robust HA upgrade in machine agent

If upgrading mongo to HA only partially
completes such that the replicaset is not
initiated, then we would previously failed
on the second attempt. Make sure we use
"Direct" connections when upgrading to HA.

Possibly fixes https://launchpad.net/bugs/1334273