Removed priority field for tasks

Removed priority field for each task, because
it can confuse the user, he already has stages
and there is no need to have another entity,
to define the order of several tasks within
a single stage we can use the order which the
tasks were defined in, e.g. Ansible uses the
same way, to define the order of tasks.

Change-Id: Ie0539266a0339b2bbf2857c88694d0a691338ec6
Implements: blueprint cinder-neutron-plugins-in-fuel
Merge "Removed priority field for tasks"