labs: add Ubuntu 14.04 LTS support library

This patch adds an osbash support library for Ubuntu 14.04 LTS.

It allows osbash to build basedisks based on that distribution.

The patch also documents the new option in localrc.

Note that the new library differs from the Ubuntu 12.04 LTS version only
in the URL of the ISO image. The URL could be passed as an argument and
both libraries merged. Such a refactoring can be undertaken later on,
should we keep adding libraries we almost identical content.

Also note that these libraries do not pass bashate (formerly know as
bash8). Maybe we can find a better way to store the boot argument string
that is bashate conformant, readable/editable, and correct code.

Implements: blueprint openstack-training-labs
Change-Id: I2a6f848f47e8bcb5a9ff7bb3abb5cbef9d7790a5