Complete rework of lxc-fedora template

Complete rework of lxc-fedora template

Heavily refactored version of the Fedora container creation template.
Removed compatibility to unsupported Fedora releases including sysvinit
and yum stuff. Added new commandline arguments found in other templates:

  --mirror   : To set custom HTTP(s) Fedora download mirror
  --packages : List of custom packages to install into a new container
  --debug    : Run with shell script with 'set -x'

There are also some new environment variables which can be used to
customize the template behaviour. See --help output.

Supports Fedora >=24. By default Fedora 25 will be installed except on
Fedora hosts, where the host release is taken. To simplify the code
path (or at least not make it more complex) all non-Fedora hosts will
now use the LiveOS-image based bootstrap environment even when they
would natively support rpm, yum or even dnf (e.g. Gentoo, CentOS).

Mainly runs systemd services by default (journald, networkd, resolved
logind).

Signed-off-by: Reto Gantenbein <reto.gantenbein@linuxmonk.ch>