Tolerate older agents not knowing "prepare" message

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Send 'prepare' to agent before 'stop'

This means the job's bits are downloaded before the job is stopped
reducing the amount of time the job spends in a stopped state. This only
works with agents that understand a 'prepare' message. We suppress
errors from older agents that do not understand this message.

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Prefer single quotes for uninterpolated strings

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>