Fix default value of Base and Target release and dir
Fix default value of Base and Target release and dir

BASE_RELEASE and TARGET_RELEASE var are used for respective dir and
correct value for them is 'old' and 'new' which are nothing but the
dire for base and target devstack.

Also correct the default valeu for BASE_DEVSTACK_BRANCH which will be
updated on every cycle.

Change-Id: Ia2b50f3e48921f81fe12cd94347e1363b29681ea

openstack-dev/grenade,openstack-dev/grenade