Mark record_type in PaaS Event Format doc as optional
Mark record_type in PaaS Event Format doc as optional

No record_type in the samples, required record_type cause confusion.

Change-Id: I2c8a104537455621d1c5a3c7ac9dc356627d448f
Closes-Bug: #1368824

ityaptin/ceilometer,cernops/ceilometer,isyippee/ceilometer,redhat-openstack/ceilometer,eayunstack/ceilometer,fabian4/ceilometer,mathslinux/ceilometer,idegtiarov/ceilometer,eayunstack/ceilometer,ityaptin/ceilometer,openstack/ceilometer,fabian4/ceilometer,redhat-openstack/ceilometer,mathslinux/ceilometer,openstack/ceilometer,cernops/ceilometer,idegtiarov/ceilometer,maestro-hybrid-cloud/ceilometer,maestro-hybrid-cloud/ceilometer,isyippee/ceilometer