vlog: Ability to override the default log facility.

vlog: Ability to override the default log facility.

When Open vSwitch is run in hundreds of hypervisors, it is
useful to collect log messages through log collectors. To
collect log messages like this, it is useful to log them
in a particular RFC5424 facility in the local system. The
log collectors can then be used to collect logs anytime
desired.

This commit provides a sysadmin the ability to specify the
facility through which the log messages are logged.

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