[cta-frontend] Adds cta.log.level in config file

Default CTA Frontend log level is changed to INFO. To set it back to
debug level, add this line to /etc/cta/cta-frontend-xrootd.conf:

cta.log.level DEBUG

Valid log levels are EMERG, ALERT, CRIT, ERR, WARNING,
NOTICE (==USERERR), INFO, DEBUG