Support multiple agents to wait for

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
update code based on comments

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
set default signature to None, test for none in body

Signed-off-by: Adam Stokes <adam.stokes@ubuntu.com>
Merge pull request #152 from Ubuntu-Solutions-Engineering/ex-multi-wait-agent-state

Support multiple agents to wait for