This will allow to use '-e docker_version=1.12' in ansible playbook
execution. It's also backward-compatible and will work with floating
docker_version format in custom yaml files.
Closes#702
On CoreOS where there is no package management, perform zero-trip
loops instead of throwing an exception for iterating over a member
of an undefined variable.
Alexander Block
Aleksandr Didenko
Stig Telfer
Antoine Legrand
Smana
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