Use absolute path in nodepool_template default value

This change allows using absolute paths for 'nodepool_template',
instead of assuming that the templates will always live within
'openstack_project/manifests/nodepool'.

Change-Id: I480188d23b338c4011fd614c7377cb4c7e244ab5
Closes-Bug: 1383719
Merge "Use absolute path in nodepool_template default value"