Allow to set matching_metadata with the cli

This change allows to set the matching_metadata of a alarm like this:

 ceilometer alarm-create --matching-metadata 'key=value' \
      --matching-metadata 'key2=value2' --name 'alarm' ...

Fixes bug #1201877

Change-Id: I22bf261b0a9580a06ae107ed45d082171f21fcc4