kdump: Try to resolve kdump destination host

Tries to resolve kdump destination host before modifying kdump.conf. If
destination address is not resolvable, exception it thrown.

Change-Id: Ibb5e83769ec9d6511d90f0e4f9b218338bb5d99a
Bug-Url: https://bugzilla.redhat.com/1131161
Signed-off-by: Martin Perina <mperina@redhat.com>