Adding install-server::ubuntu-mirror class

Adding forgotten ubuntu-mirror class. Add tests and rspec tests for the
new class. Reorganize a bit the role class so that a needed user is
created before the ubuntu-mirror class is realized
In the process also rename role::install-server classes to
role::installserver to avoid any potential scoping problems

Change-Id: I382cb74b3405289613dd62481934e6d215618d73