Fix service groups with zookeeper

Service groups using zookeeper don't work due to apparently improper
handling of the zookeeper session creation.

No additional unit tests as the mocking these interfaces probably
doesn't actually provide substantial future guaruntees. The correct
long term fix is to enable zk for unit testing upstream.

Change-Id: I21691843cb4936d10a0b82df41aef3afe5bf2519
Closes-bug: #1239864
Merge "Fix service groups with zookeeper"