Add bootstrap options

[combineads]

What this PR does / why we need it:

We need additional java options in bootstrap.conf to set up proxy for external internet access.

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• [x] Variables are documented in the README.md

[banzo]

Thank you very much, do you mind adding some documentation for that variable in the README table, maybe with a pointer with NiFi doc?

[combineads]

I've added that variable to the README.