Add wait to DFDaemon class.
Add wait to DFDaemon class.

Add the method wait to the DFDaemon class. This will allow threads to
wait for one another to finish.

This is useful in case one threads needs to wait for another thread to
finish a task before continuing.

Change-Id: I6972477f38e1980d0a8bfea6072d2d5410872fef

FrankDuan/df_code,openstack/dragonflow,openstack/dragonflow,openstack/dragonflow,FrankDuan/df_code,FrankDuan/df_code