Cleanup get_metrics method.

Formatting only happens on a single result, and association between
field and label is now easier to maintain.
Change wording for variables in `get_metrics`.

Everywhere else uses table(s) to reference which tables data is coming
from. Creatting an intermediate variable `mapping` makes the list
comprehention a little clearer.
Consume errors when querying Cass. for metrics.
Change `get_metrics` to query correct table.

`scaling_config` was changed over a month ago to `scaling_group`.
Reflecting this in `get_metrics` and tests.
Merge pull request #377 from rackerlabs/cleanup-get-metrics

Cleanup get_metrics method.