lava-dispatcher appears to require root permissions (not documented)

lava-dispatcher uses sudo to execute shell commands, e.g.:

def image_partition_mounted(image_file, partno):
...
    mount_cmd = "sudo mount -o loop,offset=%s %s %s" % (offset, image, mntdir)
...

However, once that partition is mounted, it tries to access it with direct file access calls, e.g.:

    def _customize_ubuntu(self, rootdir):
        self.deployment_data = Target.ubuntu_deployment_data
        with open('%s/root/.bashrc' % rootdir, 'a') as f:
            f.write('export PS1="%s"\n' % self.deployment_data['TESTER_PS1'])

of course, that files, as that file in mounted partition is not accessible to normal user, but requires root privs.

So:
1. There appears to be some machinery to allow lava-dispatcher to run as non-root user, but it's incomplete.
2. Current docs as present at http://lava-dispatcher.readthedocs.org/en/latest/ don't document root requirement.

Point 2 apparently would the easiest way to fix this. Longer term, it probably would be nice to complete non-root support though, given that there're already partial measures for that.