deeply validate ganglia::gmetad's clusters parameter

This should address the confusing template error reported in #12 when
incorrect data is passed to this parameter.
add docstring to ganglia_validate_clusters() function
make ganglia_validate_clusters output consistently testable

Between Ruby 1.8.7 and 1.9.3+
Merge pull request #14 from jhoblitt/feature/validate_gmetad_clusters

Feature/validate gmetad clusters