Handle SIGHUP in ovs neutron agent

Handle SIGHUP in ovs neutron agent

This change allows ovs neutron agent not to die on receiving SIGHUP
and reload its logging options.

Note that this patch allows changing only logging options.
All other config options are not handled explicitly and changing
them using SIGHUP can lead to unpredictable circumstances.
So, until changing other options is handled it is highly recommended
to use SIGHUP for changing ONLY logging options.

Change-Id: Ic0cf8a9ca7f3a16b556a6825e2979471ae136c33
Partial-Bug: #1276694