This change allows the deployer to specify lists of distro packages
which will be installed in addition to those specified by this role.
Rather than simply concatenate the lists of installed packages, extra
tasks are added to make the install ordering be as follows:
* install initial packages for repo/gpg keys
* configure repos
* install user defined extra packages
It is possible to install packages from the newly configured repos, as
the user defined package list is installed last.
Change-Id: I86e541e0c7d38460c697807c4f6ae5b6e7310a15
a72e8b9aac