Convert value type in alarm matching_metadata

Commit I0667db868c6f827867a5a20e4a3fa22fcad1a6a1 added query
property in OS::Ceilometer::Alarm resource and add
matching_metadata elements to query list. Value type in
query elements need to be a string, but value type in
matching_metadata elements can not be a string. So we need to
convert value type to string, when we add element to query list.

Change-Id: Ie6af0bfca4f3e089e0ee3e226804a2d365f6d6ae
Closes-bug: #1380168
Merge "Convert value type in alarm matching_metadata"