The RBD driver and almost all backup drivers rely heavily on eventlet's
tpool, which has a default of 20 threads, which will be too low.
Currently the only way to change this is using the environmental
variable EVENTLET_THREADPOOL_SIZE, which isn't very clean for the
openstack services.
This patch adds the possibility of setting specific values for each
backend and for the backup service, and increases the default for the
backup service from 20 threads to 60.
The backup service can be configured under [DEFAULT] section with option
backup_native_threads_pool_size, and the backends under their specific
sections with backend_native_threads_pool_size.
Change-Id: I5d7c1e8d7f1c6592ded1f74eea42d76ab523df92
Closes-Bug: #1754354
