Syslog support for neutron metadata proxy

Syslog support for neutron metadata proxy

When neutron metadata proxy reads the cli args
which are propagated from its parent processes
(l3-agent and dhcp-agent), if log-dir and log-file
are not defined in the context, it will also
double-check whether syslog is enabled and
its facility in the user-defined configuration
files (/etc/neutron).
As a result, the log behavior of each neutron
component becomes consistent.
File log is the default logging function. The syslog
is enabled when use_syslog is explicitly defined and
file log options are all disabled.

Fixes: bug #1184500
Change-Id: I7804fa8794071d21513e068d084607691215522f