Fix the mapbox for Druid

[x4base]

Currently when we try to use mapbox on a Druid datasource, it will show 'Invalid Choice' error of SelectField and "query() got an unexpected keyword argument 'columns'" due to the inconsistent interface between SqlTable and DruidDatasource. With this patch, mapbox still won't work on Druid, because you can not select columns in Druid queries. But at least it helps reducing some errors.

If I want to move one step forward, what can I do to make it work on Druid? Should I use metrics_combo as the choices of all_columns_x and all_columns_y instead of column_names?

[coveralls]

Coverage remained the same at 81.036% when pulling 0e5b5cd on x4base:fix_mapbox into 8135c24 on airbnb:master.

[x4base]

OK, I've just noticed the dicussion in #707. Does it mean this is not needed because mapbox is going to be disabled?

[mistercrunch]

For the record, @LAlbertalli isn't a core committer, he's turning the feature off in his own production deployment. We're committed to the feature but it may need some tweaks.

To make the feature work for Druid, we need to implement the columns feature of the query interface to perform a Select type query (unaggregated) against druid. I started it in a branch and got stuck along the way, it seemed like PyDruid had issues with Select queries and didn't get to the bottom of it.

[LAlbertalli]

@mistercrunch , yes, sorry if my statement was misleading. I was speaking of our internal branch.