support different domain/forest levels - use powershell on 2012

By default, 2008r2 will use 2008r2 forest and domain levels, and 2012
will use 2012 domain and forest levels - use AD_DOMAIN_LEVEL and
AD_FOREST_LEVEL to change
powershell on 2012 provides all of the command line utilities needed for
unattended AD and CA installation and configuration, so use them instead
of the hodgepodge of commands and config files used on 2008r2