Add overwrite_mode to forklift config map

[bmitchinson]

Ticket Link #760

Description

Leaving as a draft until base image issues are resolved.

• Add overwrite_mode to forklift config map
  • Set from system env variable. PR for this corresponding chart value

Reminders:

• If changing elixir code in an "app", did you update the relevant version
  in mix.exs?
• NA If altering an API endpoint, was the relevant postman collection updated?
  • NA If a new version of smart_city is being used (new fields on a struct), were the relevant postman collections updated?

[LtChae]

I have concerns about the readability of this. Notnot in front of a direct environment variable call is a bit opaque. Consider a case block that clearly lays out the possible values of the variable and assigns a boolean value based on it

[bmitchinson]

204b775

Corrected this in the above commit, good advice. Requesting reapproval.