Include jetty-servlets as dep for use with filter framework

Include jetty-servlets as dep for use with filter framework

jetty-servlets is super useful to do things like DoS and QoS filtering
in the context of a J2EE servlet.  This patch adds a dependency on
jetty-servlets (whatever version is in odlparent) to allow easy
configuration and specification of DoS/QoSFilter.  To enable DoSFilter
for example, write the following to
etc/org.opendaylight.aaa.filterchain.cfg:

customFilterList=org.eclipse.jetty.servlets.DoSFilter
org.eclipse.jetty.servlets.DoSFilter.maxRequestsPerSec=1
org.eclipse.jetty.servlets.DoSFilter.delayMs=10000

By simply adding the dependency, it allows us to use filter(s) from
jetty-servlets without manually installing the bundle.  It also
allows us to avoid version skew as we are just grabbing whatever
upstream odlparent offers.

Change-Id: I63f135872c447e9be75dde0547f81d7876afe761
Signed-off-by: Ryan Goulding <ryandgoulding@gmail.com>