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.

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variables.

1. AWS has issues with ext4 (use xfs instead for CentOS only)
2. Make sure all the centos config files are include in the systemd config
3. Make sure that network options are set in the correct file by os family

This allows downstream items like opencontrail and others change variables
in expected locations.

install docker on a largest number of linux distribution (based on https://github.com/marklee77/ansible-role-docker)