[config-ref] Add parallel_read_safe to ext.remote
[config-ref] Add parallel_read_safe to ext.remote

To normally work on a multi processor machine.

If this flag is not set, the below error occurs with sphinx-build:

  Warning, treated as error:
  WARNING: the ext.remote extension does not declare if it is safe
  for parallel reading, assuming it isn't -
  please ask the extension author to check and make it explicit

Change-Id: I306c8b8db27a930e860c669b32d97c3a0fbbc32f

openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals