Add new fab tasks for addition and removal of tor-agents

Added the following new fab tasks for delete of tor-agent
1. delete_tor_agent : Removes all the tor-agents in all the nodes as specified in testbed file.
2. delete_tor_agent_node: Removes all the tor-agents in the specified node. Details of tor-agent are picked from testbed file.
3. delete_tor_agent_by_id: Removes the specified tor-agent (identified by tor-id) from the specified node. Details of tor-agent are picked from testbed file.
4. delete_tor_agent_by_index: Removes the specified tor-agent (identified by index/position in testbed file) from the specified node. Details of tor-agent are picked from testbed file.
5. delete_tor_agent_by_index_range: Removes a group of tor-agents (identified by indices of testbed file)from the specified node. Details of tor-agent are picked from testbed file.

Added the following new fab tasks for add of tor-agent

1. add_tor_agent_by_id: Adds the specified tor-agent (identified by tor-id) to the specified node. Details of tor-agent are picked from testbed file.
2. add_tor_agent_by_index:Adds the specified tor-agent (identified by index/position in testbed file) to the specified node. Details of tor-agent are picked from testbed file.
3. add_tor_agent_by_index_range: Removes a group of tor-agents (identified by indices of testbed file) to the specified node. Details of tor-agent are picked from testbed file.

Change-Id: I13ea46ff445d14280799be5d98677eaeff0f996f
Closes-Bug: 1454083, 1454093