Allow using regexp for model_map and group_map keys

[ytti]

Use case is that the source returns string which has unrelated and variant data but the actual model is included somewhere in this data

For example source could have space separated list of tags and one of these tags could be model name. Eg 'procurve switch'.

Closes #3360

[ytti]

I honestly cannot remember why I am normalising all keys to strings, but tests at least pass even if we don't normalise them.

[ytti]

Another option, which doesn't require changing the key_to_s would be to introduce new config stanza like.

source:
  map:
    model:
       "oxidized_model": "source_model"

By swapping the order, we would guarantee the key is string, and value could be potentially any other class.

How we could transition to this new config stanza is that we could check existence of old, if old is found, we could emit warning that it's being deprecated and removed later in X, potentially link to documentation. Then we swap key/value and process the old and new stanza the same way.
We could even replace the user config with new version, however this would remove their formatting and comments as we don't preserve those when we write.

I don't feel strongly.