The fabfile is now responsible to find the target nodes for each
execution context. It practically gets the corresponding component
for each role and invokes its setup() method.

To get the execution context for each action we use fabric's execute()
and update the context based on env.host.

Some things can be done concurrently (setup of each VMC node).
To do so, we use the fabric's @parallel decorator.

Additionally we support setup of a single component/role on a target
node or even the execution of a single method of a component.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>