2023-01-22 Pi-hole admin password changes - master branch - PR 1 of 3

Circa July 9, 2022 [docker-pi-hole PR 1106](pi-hole/docker-pi-hole#1106)
made some significant changes to how the admin password is handled. The
only documentation for this change was in
[Changelog 2022.07](https://github.com/pi-hole/docker-pi-hole/releases/tag/2022.07):

> Always use WEBPASSWORD env var if set by @rdwebdesign in #1106

This change was reported on [Discord](https://discord.com/channels/638610460567928832/638610461109256194/1066408650077388860).

To summarise the new behaviour:

1. If `WEBPASSWORD` is not set on first launch, a random password is
generated.
2. If `WEBPASSWORD` is not set on second-or-subsequent launch, whatever
password was already in effect is re-used. This also applies when there
is no admin password.
3. If `WEBPASSWORD` is defined but has a null value, any pre-existing
password is cleared and the Web UI bypasses the login screen.
4. If `WEBPASSWORD` is defined and has a value, that value is always
used to update the admin password stored internally.

This PR adjusts the IOTstack Pi-hole template to default to the third
option above (ie no admin password). This avoids the need for any
"build" process and the need to set/generate any password, and obviates
the need for the associated Python scripts which have been removed.

A comment is added to the service definition template directing users
to the IOTstack wiki page where the behaviour of `WEBPASSWORD` is
documented in full.

This change should only affect new users.

Any existing users who had followed the previous procedure of selecting
Pi-hole, choosing a password or allowing one to be generated, and
bringing up the container would have an admin password matching the
value of `WEBPASSWORD`.

Any existing users who had reset their password via:

```
$ docker exec pihole pihole -a -p myNewPassword
```

will either have:

1. Changed their service definition to match. The July 9 2022 changes
would already be enforcing  `WEBPASSWORD` on each launch.
2. Not changed their service definition to match. The July 9 2022
changes would already have reverted to the value of `WEBPASSWORD`.
3. Removed `WEBPASSWORD` from their list of environment variables. The
July 9 2022 changes would continue to use the existing password.

This PR also takes the opportunity to:

1. Re-align the service definition entries.
2. Adopt new-style TZ which either accepts a value from `~/IOTstack/.env`
or defaults to `Etc/UTC`.
3. Add a comment pointing to the Pi-hole documentation for its supported
environment variables.

IOTstack Pi-hole documentation updated.

Signed-off-by: Phill Kelley <[email protected]>

Author: Paraphraser