Make help for agent-update more verbose

Amended the description of the command to reflect what it
really does.
Added two optional aguments:
    * admin-state-down - sets admin state up of the agent to false
    * description - provides the description for the agent

Added a unit test.

Change-Id: Ie9f30f30102bef6735dc9ed538eca9a1183c2780
Closes-Bug: #1254241