Add generic 'list' target

Usage:
 make list
Make Makefile more generic for building roles

This is done in order to prevent users from editing the Makefile
for every new role.

It could be used like the following:
- make chef-server INST=/tmp/edeploy
=> in this case 'chef-server' is build on top of 'base'

- make devstack INST=/tmp/edeploy BASE=openstack-common
=> in this case 'devstack' will be built on top of
'openstack-common' which is itself built on top of 'base'.

Known limitations: it only supports one base iteration. It is not
possible to have role1 -> role2 -> role3 -> base