Description of the problem:
beah-beaker-backend fails to set HOSTNAME configuration variable by default and emits a warning message.

Version-Release number of selected component (if applicable):
systemd-208-11.el7.x86_64
beah-0.7.3-1.el7eng.noarch

How reproducible:
Always

Steps to Reproduce:
1. Run a recipe in beaker
2. Check the output of "systemctl status beah-beaker-backend"

Actual behavior:
# systemctl status -l beah-beaker-backend | grep -i warning || echo OK
Jun 09 01:06:38 myhost beah-beaker-backend[1839]: --- WARNING: Value for DEFAULT.HOSTNAME (None) is not an string.

Expected behavior:
# systemctl status -l beah-beaker-backend | grep -i warning || echo OK
OK

Additional Information:

beakerlc.py tries to obtain hostname via "HOSTNAME" env:
  def defaults():
    ...
    d.update({
            ...
            'HOSTNAME':os.getenv('HOSTNAME'),

However, os.getenv('HOSTNAME') returns empty.

Using the output of '/bin/hostname' instead solves the problem.
Or there might be better systemd-way to get the equivalent information.