Output interactive mode env to a file

[kmazurek]

Resolves #518

This change makes interactive mode output its environment variables to a file (which can be sourced), rather than print them out to the console. Example of the file's contents:

export PATH=/tmp/tmpx7o16vhf:$PATH
export YAGNA_APPKEY=a70facb9501d4528a77f25574ab0f11d
export YAGNA_API_URL=http://172.19.0.6:6000
export GSB_URL=tcp://172.19.0.6:6010

Known limitations

• there's no support for Windows, adding it would significantly increase complexity due to how PATH extensions are handled
• each time the file is sourced the gftp script directory is appended to the PATH variable
• subnet tag currently cannot be set through an env variable (perhaps it should be possible?) added to yapapi in Make engine params configurable via env variables yapapi#546

Possible additions

• revisit/improve extending the PATH variable done
• add an integration test for the interactive mode done
• mention the "new way" in readme/docs done

[azawlocki]

I would be really grateful if you left the old way of starting the requestor script as an alternative to source'ing the .env file.

[azawlocki]

LGTM!