Add CURL variables to slugbuilder appConfig k8s_util

[Cryptophobia]

We should add CURL_TIMEOUT and CURL_CONNECT_TIMEOUT to be configurable per application on the slugbuilder ENV vars.

https://github.com/heroku/heroku-buildpack-ruby/blob/cebdf6889c86fde4a22d58665e23c2b22e6a288b/lib/language_pack/fetcher.rb#L44

This way we will be supporting latest configuration for buildpacks for Heroku. 👍