Add support for vendor customisations in install script

Currently apt repos are hardcoded to come from upstream ubuntu
and the github account is hardcoded to stackforge. In order to
support alternative distributions using this data model, this
patch adds a variable called $vendor that can customise these
according to a particular distributor's preference.

Change-Id: Iba4353bc226b973ce1ad0c84e0f670b11f23bdf2
Merge "Add support for vendor customisations in install script"