Unexpected response in agent-list command

Problem with 'alive' field. if you try this command you obtain an
unexpected response:

$ neutron agent-list --field agent_type
'alive'

The problem is that the 'alive' field is not checked for instanciate
before to check its value. Under the hood, there is a non catched
Keyerror exception converted to unicode.

Change-Id: I59b313494605f64c5588324d1153614b1c11920f
Closes-Bug: #1271948