commands: make add appliance work in early installation phase

this command is called at an early stage while populating the
database and there are not host defined so getHostAttr was
failing because localhost was not defined.

Let's just use global attribute here to fetch
Kickstart_PrivateKickstartHost
commands: add rocks exceptions class and use them

Added HostnotFoundException to be used all the time we don't find
the requested host