option references should not contain newlines

[Vincent Ladeuil]

Encountered while debugging udd config migration.

The regexp used to match option references allows '\n' which makes no sense for an option identifier and fails to expand constructs using '{' and '}' like:

logrotate_configuration='''{pi.driver.log.debug} {
    daily
    dateext
    rotate 7
    delaycompress
    compress
    notifempty
    missingok
}
'''

The fix is trivial and I'll submit it shortly.