Scheduler rpcapi 2.9 is not backwards compatible

Patch 552693e4ad51291c8bfd28cd1939ed3609f6eeac which added scheduler
rpcapi version 2.9 does not provide any backwards compatibility.

This patch adds the check using 'self.can_send_version', and falls
back to the latest Grizzly compatible version if it the version is
capped.

Also takes the opportunity to make the docstring more consistent in the
scheduler rpcapi SchedulerAPI class.

Closes-bug: #1220112

Change-Id: I5a1e08c1066cae00d47def45f7b87846f679cbbe
Merge "Scheduler rpcapi 2.9 is not backwards compatible"