non-interactive mode leaves broken dispatcher configuration

[Antonio Terceiro]

after installing a lava instance, I ended up with dispatcher configuration that had mostly empty variables:

LAVA_SERVER_IP =

# Location for rootfs/boot tarballs extracted from images
LAVA_IMAGE_TMPDIR =

# URL where LAVA_IMAGE_TMPDIR can be accessed remotely
LAVA_IMAGE_URL = http://%(LAVA_SERVER_IP)s/images

# Location on the device for storing test results.
LAVA_RESULT_DIR =

# Location for caching downloaded artifacts such as hwpacks and images
LAVA_CACHEDIR =

# This is the address and port of cache proxy service, format is like:
# LAVA_PROXY = http://192.168.1.10:3128/
LAVA_PROXY =

the dispatcher configration code sets sensible defaults to most of these, so it should not ke skipped.