host-deploy: add IPTablesConfigSiteCustom vdc_option

enables adding site specific rules without overriding entire
configuration.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1111513
Change-Id: I713a7886216c0e868fc4bc1e77077bab6548108f
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>