fix the bug that gnocchi dispatcher can't process single sample
fix the bug that gnocchi dispatcher can't process single sample

When we use UDP to publish samples to Ceilometer-collector, it
always come one by one. So we have to check whether the gnocchi
dispatcher receive a single sample or a list.

Change-Id: Idb19c8b94681b827fdfcef9c0bbbf3b85688b23c
Closes-Bug:#1523124

ityaptin/ceilometer,openstack/ceilometer,openstack/ceilometer,idegtiarov/ceilometer,idegtiarov/ceilometer,ityaptin/ceilometer