Fix security issue with propagation to DNS alias

Facility manager can create destination to any free hostname (or IP
adress). The "free" means that name is not used on other facilty as a
destination or host's name. Exception from this is when the manager is
also facility manager on such facility.
Therefore any facility manger could create destination to any machine
(using IP address on other alias) and create resources with any services
(like sshkeys_root or k5login_root) and Perun will propagate there the
configuration.

Perun-slave now contains two variables DNS_ALIAS_WHITELIST and
FACILITY_WHITELIST which contains from which destinations or
which facilities the perun-slave accepts new configuration.
By default slave only accepts configuration from `hostname -f`.
Add log_debug function to perun slave script

If $DEBUG env. variable is set then this function logs to syslog.
Otherwise it does nothing.