fix optional parameter of creating sample

When creating sample without parameter "meter_name" in ceilometer CLI,
The result is 'counter_name' ,the parameter "meter_name"(i.e.counter_name)
isrequired,but it is not in the help messages of "ceilometer sample-create".

Change-Id: I17ee19354b51cd5095677438359d036783d82b15
Closes-bug: #1267378
Merge "fix optional parameter of creating sample"