Starting with Bundler 2.1, the `--path`, `--without`, and `--deployment`
options are deprecated. In other words you are no longer supposed to
specify these when running `bundle install`.

Instead, Bundler wants you to set these options _externally_ from the
install command. That way all subsequent invocations of bundler can use
the same external configuration without you having to remember which
flags to use each time.

There are two ways to specify this external configuration: providing
environment variables, or running `bundle config`. This commit
implements the latter.

To summarize, prior to running `bundle check`, `bundle install`, or
`bundle clean`, Capistrano will now run a new `bundler:config` task.
This task executes the following command as many times as needed:

```
bundle config --local KEY VALUE
```

Each execution sets the external Bundler configuration KEY to VALUE.

The following Capistrano variables are automatically consulted to get
theses KEYs and VALUEs:

- :bundle_gemfile
- :bundle_path
- :bundle_without

This commit also introduces a new variable:

- bundle_config

It is a Hash that can contain any arbitrary KEY and VALUE pairs to send
to `bundle config`. By default it has a single entry:

```
set :bundle_config, { deployment: true }
```

Finally, this commit removes `--deployment` option from the default
value of `:bundle_flags`, since that flag is deprecated. It has been
replaced by the default `:bundle_config` as mentioned above.

…1-support