fuel: Switch using labels instead of slaves

fuel: Switch using labels instead of slaves

This change switches fuel jobs to use labels instead of tying
jobs to slaves.

The possibilities and changes introduced with this approach are
- If things do not work as expected, slave-label parameter can
  be set to certain slave name to get back to old way of
  tying jobs to slaves.
- If a scenario requires a different slave, it can still be
  created using baremetal or virtual but the label can be
  different, pointing to a slave that is capable of running
  that scenario. (fuel-baremetal-multisite, etc.)
- The auto-trigger-name for brahmaputra is moved under the
  branch anchor and can be used like this if triggers for all
  all the jobs for a certain branch need not to be triggered
  automatically but job is still kept enabled in case if some
  crucial bug needs to be tested.
- No of max concurrent builds stepped to 4 in order to use
  all the labelled slaves available. This will require some
  adjustments before colorado jobs start to run in order to
  share resources between branches fairly.
- All the fuel CI PODs except lf-pod1 have been removed.

Change-Id: I91eca6743bb3d34263e634f9b74550efa1e2ddbe
Signed-off-by: Fatih Degirmenci <fatih.degirmenci@ericsson.com>