Pulling docker image from registry

Changes are such that on executing setup.sh docker image is pulled from container registry and if fails it builds a local image
Readme is updated with steps on how to login to registry(one time process)

Change-Id: Iaf6d6539bf6adca86c750ccfd26cb874cd32cf9d
Signed-off-by: Annam Sai Manisha <annam.saimanisha@arm.com>