Provide explicit help string of resource-metadata

Currently, the "--resource-metadata" in ceilometer sample-create command
is not easy to use. For compatibility, this patch just provides an
explicit help string for the parameter.

Change-Id: I6d68d5b653c3b33808cba6f9a781301fba9b44e3
Closes-Bug: #1324759
Merge "Provide explicit help string of resource-metadata"