Make default limit for search configurable
Make default limit for search configurable

Currently the default limit is hardcoded to 10, which makes it
inconvenient when we want to change to other value. So make the
default to CONF.limit_param_default.

Change-Id: I709533f677f37b90dc2038e1846e66c05ea42d3e
Closes-Bug: 1549018
openstack/searchlight,openstack/searchlight,openstack/searchlight