This allows Ubuntu/Debian to downgrade Docker version if
a newer version is installed, instead of failing.

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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)