Failed to execute concurrent subscription-manager register commands

Issue

Deploying or upgrading multiple servers can lead to a situation where concurrent subscription-manager register actions are executed.

This is breaks two scenarios:
- upgrading RHEL6.5 hosts to the latest build, because the clients need a subscription-manager re-register to make sure that the latest repository configuration from Satellite 6 is retrieved. Doing concurrent upgrades on multiple hosts will give timeout errors on many hosts. A sequential action will take a lot of time much time.
- unattended automated testing from a CI environment where test servers are kickstarted and re-registered without human intervention.