Add support for Ubuntu 16.04

[andreasnauerz]

I have been told that we do not yet support Ubuntu 16.04.
If this is really the case this should be fixed as 16.04 is out since quite a while and more and more people will switch over from 14.04 to 16.04.

[csantanapr]

For whom ever wants to contribute this, this is the recommended approach.

1. Support both ubuntu14 and ubuntu16

Make the following scripts work out of the box on unbuntu14 or unbunt16.
https://github.com/apache/incubator-openwhisk/tree/master/tools/ubuntu-setup/

tools/ubuntu-setup/ansible.sh
tools/ubuntu-setup/docker.sh
tools/ubuntu-setup/java8.sh
tools/ubuntu-setup/misc.sh
tools/ubuntu-setup/pip.sh
tools/ubuntu-setup/scala.sh

2. Version of docker engine 1.12

Currently we only support docker 1.12
Update tools/ubuntu-setup/docker.sh that by default install docker 1.12 on both (ubuntu14/16), for those that want to experiment on their own, allow via environment variable to override this and instead use latest version of docker.

3. Have a way to test it

I think there could be 2 options:

1. Setup Travis to run in matrix running in parallel ubunt14 and ubunt16
2. Update Vagrant deployment to use ubunt16