Adding support for Docker service 'After' attribute in Unit file

[eyal-lupu]

Hello

My environment (but this should be environment agnostic):

O/S: CentOS 7
Docker 1.12.2

The docker-engine RPM sets up a systemd unit file with After=network.target option. While this is usually enough for most cases it break on our environment as we require some other units to start.

The enhancement request in here is to add support to this Puppet module so one could configure alternate 'After' option in the unit override file (override files already exist as part of this module so we just need to add the option).