validate hostname before adding new hosts

this patch add an extra validation step when adding new hostname
it verify that the hostname is not in the appliance names
and it is not in the form of rack<number>

In fact once a node is added with that name it can not be delete (
e.g.
if you add
rocks add host compute
to delete that host the only way is
rocks remove host compute
which will delete all the compute nodes
login appliance should be managed