Add support to ubuntu 16 #4

[micafer]

The roles as it is supports correctly ubuntu 16.
It is only required to avoid the release test.

[rfc1459]

Removing the guard for Ubuntu 14.04 isn't enough: the systemd unit shipped by Docker ignores /etc/default/docker, so all configuration flags that should be set by this role aren't used by anything.

Adding full support for Ubuntu 16.04 requires creating a new systemd drop-in, since the existing one explicitly uses EL-specific paths (could be refactored, though).

[maricaantonacci]

As pointed out by @rfc1459 removing the control on the Ubuntu version is not enough. I will work on the support for Ubuntu 16.04 adding the needed patches

[maricaantonacci]

Support implemented as per #6