scripts: rework environment variables setup (#2716)

* outsourcing env variable setup

This commit contains major parts of the work of refactoring the setup
and usage of environment variables. It outsources the setup into its own
script and provides dedicated functions to be executed at a later point in time.

A **new** env variable was added: `USER_PROVISIONG` which provides a
better way of defining which method / protocol to use when it comes to
setting up users. This way, the `ENABLE_LDAP` variable is deprecated,
but all of this is backwards compatible due to a "compatibility layer", a function provided by the new variables script.

This is not a breaking change. It mostly refators internal scripts. The
only change facing the user-side is the deprecation of `ENABLE_LDAP`. We
can prolong the period of deprecation for this variable as long as we
want, because the new function that ensures backwards compatibility
provides a clean interface for the future.

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>
Co-authored-by: Casper <casperklein@users.noreply.github.com>

docs/content/contributing/coding-style.md

@@ -130,9 +130,7 @@ function _setup_postfix_aliases
 
     cp -f /tmp/docker-mailserver/postfix-virtual.cf /etc/postfix/virtual
 
-    # the `to` is important, don't delete it
-    # shellcheck disable=SC2034
-    while read -r FROM TO
+    while read -r FROM _
     do
       # Setting variables for better readability
       UNAME=$(echo "${FROM}" | cut -d @ -f1)