Customize pihole-FTL.conf with FTLCONF_ prefixed environment variables.

[AlexandreRatte]

Description

Parse all environment variables starting with the FTLCONF_ prefix and setup the variable's suffix as a pihole-FTL.conf customized setting. For example, setting the FTLCONF_REPLY_ADDR4=10.1.2.3 environment variable would result in a pihole-FTL.conf setting of REPLY_ADDR4=10.1.2.3.

This PR is marked as having a breaking change since the yet undocumented ServerIP and ServerIPv6 variables have been deprecated in favor of provisioning them as FTLCONF_REPLY_ADDR4 and FTLCONF_REPLY_ADDR6 respectively.

Motivation and Context

Addresses #922 and #919

There were use cases where pihole-FTL.conf could be supplied through a read-only filesystem i.e.: a Kubernetes ConfigMap, and calling the changeFTLsetting function on such a read-only file would cause the container to crash loop. This PR is meant to provide the pi-hole administrator all the flexibility that was previously associated with volume-mounting a customized pihole-FTL.conf, but now by using environment variables.

How Has This Been Tested?

• docker run -it -e FTLCONF_REPLY_ADDR4=10.1.2.3 -e FTLCONF_MAXDBDAYS=7 issue922
• Shelled into the running container then executed:

# cat /etc/pihole/pihole-FTL.conf
PRIVACYLEVEL=0
MAXDBDAYS=7
REPLY_ADDR4=10.1.2.3
#

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[PromoFaux]

I've not tested this functionally, yet, but I like the idea a lot. I am wondering (out loud) if we can do something similar for setupVars.conf keypairs, so SETUPVARS_ env variables, and what this would look like.

I also need to check a few things, but we might be able to keep the ServerIP ones around so that this does not become a breaking change (although I'm all for breaking changes if they are good and sensible like this)

[AlexandreRatte]

Yes, implementing something similar for setupVars.conf would probably clean things up rather nicely. However, in light of #922 causing issues to folks who are supplying pihole-FTL.conf through a read-only volume mount due to the lack of customization capabilities by other means, I would suggest attacking setupVars.conf outside of this PR.

Let me know of your thoughts, I'm happy to discuss and make amendments per your review.

[PromoFaux]

a