deploy: Delete old Fuel env if present

deploy: Delete old Fuel env if present

In order to clean up old partitioning and boot entries on target
nodes, before removing the Fuel Master, try ssh-ing into it and
removing all environments/nodes.

This is especially important for EFI systems, where old boot
entries are left behind without a proper env delete, leading to
target nodes failing to PXE boot on Fuel Master re-install.

This change assumes that:
- all Fuel Master information is unchanged between old and new
  deploy (fuel IP, password etc.);
- Fuel Master is up and running, in a sane state (target nodes
  are also online), when deploy starts;

Closes: ARMBAND-51

Change-Id: I7b61124ea9182ce2d599adcdedef03e0d8d67ed6
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>