Fix sample parameter of agent API

The parameter of agent update API has been changed from "para" to
"agent" since Nova v3 API, and this patch fixes the sample doc.

Partially implements blueprint v3-api-specification

Change-Id: Ic9c0f11a637e48daa9457ab9ae923fefc0fb350d
Don't log stacktrace for UnexpectedTaskStateError

An UnexpectedTaskStateError is actually an expected behavior and we
don't need to log a stacktrace every time it happens. This stacktrace
is one of the most common ones found in nova-compute during gating jobs.

Change-Id: I8b33f72dc0f4beaca969de084481d66987faa974
Merge "Fix sample parameter of agent API"
Merge "Don't log stacktrace for UnexpectedTaskStateError"