Implemented knife cloud openstack server delete command
add knife-cloud dependency.
update service and command classes to match knife-cloud base changes as per review.
remove dead code post review changes.
Merge remote-tracking branch 'origin/OC-8570-server-delete' into oc-8569-server-create

Conflicts:
	knife-openstack.gemspec
	lib/chef/knife/cloud/openstack_service.rb
common helpers used across commands.
use abstracted functionality.
fix command class as per review comment.