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>

target/scripts/helpers/dns.sh

@@ -17,6 +17,9 @@ function _obtain_hostname_and_domainname
   # TODO: Consider changing to `DMS_FQDN`; a more accurate name, and removing the `export`, assuming no
   # subprocess like postconf would be called that would need access to the same value via `$HOSTNAME` ENV.
   #
+  # ! There is already a stub in variables.sh which contains DMS_FQDN. One will just need to uncomment the
+  # ! correct lines in variables.sh.
+  #
   # TODO: `OVERRIDE_HOSTNAME` was introduced for non-Docker runtimes that could not configure an explicit hostname.
   # Kubernetes was the particular runtime in 2017. This does not update `/etc/hosts` or other locations, thus risking
   # inconsistency with expected behaviour. Investigate if it's safe to remove support. (--net=host also uses this as a workaround)