[KNIFE-240] use chef search to figure out instance id from node name and vice versa

http://tickets.opscode.com/browse/KNIFE-240

it is cumbersome to delete instances using ec2 instance id. this pull request enables the `knife ec2 server delete `  command to fetch ec instance id if only chef_node_name option is passed (i.e.e name_args is empty) and vice versa (i.e.e retrive chef node name when only ec2 instance id is passed along with --purge)