Changing the default of bundle config set to be --local instead of --global

[eregon]

Describe the problem as clearly as you can

bundle config set key value is currently --global by default (stored in ~/.bundle/config), while I think most usages would benefit from being local.

For instance, typical mistakes are bundle config path vendor/bundle, bundle config without db, etc.

Note also that the very similar command git config key value defaults to --local.

I think it would be much safer and intuitive to change bundle config set key value to also default to --local.
Most of Bundler configuration is project-specific and doing it globally makes often little sense, especially when not stating explicitly one wants it to be global.

Extracted from #3916

[eregon]

I think the user's expectation is that running pretty much any bundle command in a project with a Gemfile only affects that project, and nothing else.
I think system-wide changes such as global config should always be explicit, as otherwise they can be very confusing.

[eregon]

Thanks for fixing this!