vlog: Rename the currently used term 'facility' as 'destination'.

vlog: Rename the currently used term 'facility' as 'destination'.

In OVS, we currently use the term 'facility' to mean the place
where we log (syslog, console or file). In Linux's syslog() and
rfc5424, the term 'facility' is used to specify what type of program
is logging the message (e.g: LOG_DAEMON). This causes confusion
while reading vlog's code. This commit changes the term 'facility'
to 'destination'.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>