webadmin: cloud-init [4/6] - hostname validation class

Cloud-init integration requires hostname validation; this patch
refactors existing validation code to a reusable IValidation class.

Change-Id: Ic76913ae0b7cf64f6dd1e73fb67412ba6fce58f2
Signed-off-by: Greg Padgett <gpadgett@redhat.com>