gcp/builder: add doc for tags
gcp/builder: add doc for tags

closes #6446

youhong316/packer,youhong316/packer,bryson/packer,timsutton/packer,rnaveiras/packer,pecigonzalo/packer,grubernaut/packer,rnaveiras/packer,timsutton/packer,tas50/packer,mitchellh/packer,mitchellh/packer,bryson/packer,hashicorp/packer,ricardclau/packer,tas50/packer,grubernaut/packer,pecigonzalo/packer,mkuzmin/packer,youhong316/packer,ricardclau/packer,rnaveiras/packer,timsutton/packer,grubernaut/packer,rnaveiras/packer,hashicorp/packer,mitchellh/packer,paulmey/packer,ricardclau/packer,dave2/packer,mkuzmin/packer,mkuzmin/packer,grubernaut/packer,tas50/packer,hashicorp/packer,mitchellh/packer,ricardclau/packer,rnaveiras/packer,bryson/packer,mkuzmin/packer,tb3088/packer,tb3088/packer,ricardclau/packer,ricardclau/packer,grubernaut/packer,dave2/packer,bryson/packer,mkuzmin/packer,tas50/packer,timsutton/packer,bryson/packer,tb3088/packer,bryson/packer,youhong316/packer,paulmey/packer,dave2/packer,grubernaut/packer,dave2/packer,timsutton/packer,pecigonzalo/packer,paulmey/packer,tb3088/packer,dave2/packer,rnaveiras/packer,youhong316/packer,dave2/packer,pecigonzalo/packer,youhong316/packer,mitchellh/packer,tb3088/packer,paulmey/packer,tas50/packer,paulmey/packer,paulmey/packer,tas50/packer,hashicorp/packer,timsutton/packer,mkuzmin/packer,tb3088/packer