Allow force-delete irrespective of VM task_state

Cannot delete vm instance if duplicate delete requests are sent.

When user sends request to delete an instance, its task_state gets
changed to 'deleting' state. When an instance task_state is already in
'deleting' state and if at that moment the rabbitmq-server get crashed
by some reasons, then the instance task_state remains in 'deleting'
state and user won't be able to delete the instance forever.

At this moment, there is only one way to delete the instances, whose
task_state is in 'deleting' state, by restarting the nova-compute
services where these instances are running.

To avoid restarting the nova-compute service manually, modified the
force-delete api to allow instance deletion irrespective of
instance task_state.

Added new module to use delete_types as constants and replaced all
delete_type string occurances with new constants.

Closes-Bug: #1308342

Change-Id: I4d0e47662a80109ef9622d85455587d487e47c01
Merge "Allow force-delete irrespective of VM task_state"