Fix docker config handling in offline installer

Fix docker config handling in offline installer

This fixes adding of dns address to
/etc/docker/daemon.json to always use infra's ip address
instead of possibly using it's hostname.
Also similar change is done in handling of
/etc/resolv.conf in infrastructure.yml.
Ip address is specified by the "cluster_ip" variable placed on a host.
Inventory specifies ip for everything except resource server (just for consistency).

Change-Id: I2baa3f4c7d69b8dff109b5f8ae984134caf2af7d
Issue-ID: OOM-1573
Signed-off-by: Michal Zegan <m.zegan@samsung.com>