adds support for ubuntu init script default file [/etc/default/webvirtmgr-novnc]

allows local settings for things like user/group to run as, location of daemon, etc - without needing to modify init script.

e.g.:
```bash
>cat /etc/default/webvirtmgr-novnc
# Defaults for webvirtmgr-novnc init script
# sourced by /etc/init.d/webvirtmgr-novnc-ubuntu

#
# This is a POSIX shell fragment
#

# directory in which webvirtmgr-novnc is found
#DAEMON_PREFIX='/var/www/webvirtmgr/console'
DAEMON_PREFIX='/opt/webvirtmgr/console'

# user to run novnc as
#USER='www-data'
USER='webvirtmgr'

#GROUP='www-data'
GROUP='webvirtmgr'
```
also includes a few changes to adhere to ubuntu conventions for /run/, and some misc quoting and variable reuse modifications.