Update docs with modified workers default value.
Update docs with modified workers default value.

The default value for the worker config option is now 1,
as opposed to 0.

Change-Id: I49f05b31415b3c641abe582f915fc9ead63e84ff

redhat-openstack/glance,ozamiatin/glance,JioCloud/glance,takeshineshiro/glance,kfwang/Glance-OVA-OVF,rickerc/glance_audit,dims/glance,cloudbau/glance,akash1808/glance,citrix-openstack-build/glance,darren-wang/gl,paramite/glance,rickerc/glance_audit,saeki-masaki/glance,klmitch/glance,klmitch/glance,openstack/glance,stevelle/glance,dims/glance,jumpstarter-io/glance,akash1808/glance,stevelle/glance,ntt-sic/glance,wkoathp/glance,sigmavirus24/glance,takeshineshiro/glance,rajalokan/glance,JioCloud/glance,jumpstarter-io/glance,paramite/glance,tanglei528/glance,cloudbau/glance,kfwang/Glance-OVA-OVF,openstack/glance,saeki-masaki/glance,SUSE-Cloud/glance,SUSE-Cloud/glance,citrix-openstack-build/glance,ntt-sic/glance,redhat-openstack/glance,vuntz/glance,scripnichenko/glance,vuntz/glance,darren-wang/gl,ozamiatin/glance,rajalokan/glance,tanglei528/glance,scripnichenko/glance,wkoathp/glance,openstack/glance,sigmavirus24/glance