Add executor_thread_pool_size

Add executor_thread_pool_size

Change I36324bae39e05dde57b962948f832c22599c13d2 added
executor_thread_pool_size as a new param to oslo::messaging::default,
but that option was not updated in many modules like this one. This
means a user cannot set executor_thread_pool_size, as setting it
anywhere else will throw a duplicate declaration error.

This change update the oslo::messaging::default call accordingly, and
adds an option to set it, similar to Change
Id4a61c39ef6392a3d656952467c756be9e68de91

Change-Id: I446af549752cf4872b98e3320353bd93a8a9ab2f