Add boolean flag to disable the PaxOsgi appender

Add boolean flag to disable the PaxOsgi appender

Currently the PaxOsgi appender is on the OpenDaylight default
configurations for the logging system. It was found, during a research
of performance issues [1], that this adds pressure to the Java GC and is
a facility that is interesting to have configurable.

This patch adds the configuraton boolean `enable_paxosgi_appender` that
allows packaging OpenDaylight with this setting disabled. In addition,
due to a business requirement [2], this setting is set to `false` by
default.

[1] ODLPARENT-176
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1650576

Change-Id: I358618594a5703cc8a22575dde88e9273e2b8a16
Signed-off-by: Michel Peterson <michel@redhat.com>