update docstring for driver_periodic_task's parallel param
update docstring for driver_periodic_task's parallel param

This updates the docstring for the parallel parameter to make it more
obvious what a value of False means; that it will cause the conductor's
other periodic tasks to be blocked.

Change-Id: Idc4504fdf5b08095f52d21ad938496c88daab742

devananda/ironic,ionutbalutoiu/ironic,Tan0/ironic,dims/ironic,openstack/ironic,naterh/ironic,NaohiroTamura/ironic,naototty/vagrant-lxc-ironic,pshchelo/ironic,openstack/ironic,SauloAislan/ironic,bacaldwell/ironic,bacaldwell/ironic,pshchelo/ironic,naototty/vagrant-lxc-ironic,SauloAislan/ironic,supermari0/ironic,ramineni/myironic,NaohiroTamura/ironic,rdo-management/ironic,hpproliant/ironic,redhat-openstack/ironic,ionutbalutoiu/ironic,dims/ironic