server: Unmask association signals

PropertiesChanged signals are masked by default by pyphosphor.  When
left masked, PropertiesChanged signals are not emitted by the mapper
when it adds or removes endpoints from an association.

Unmask signals when association objects are created so the signal
is emitted.

Tested: Manually verified PropertiesChanged signals are emitted
  on association endpoint updates with busctl.
Change-Id: Ie222aa414554c416a0c7483ddbcdab1c81dac77e
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/obmc/mapper/server.py b/obmc/mapper/server.py
index 35415f8..3837a17 100644
--- a/obmc/mapper/server.py
+++ b/obmc/mapper/server.py
@@ -188,6 +188,7 @@
         """
         super(Association, self).__init__(conn=bus, object_path=path)
         self.properties = {self.iface: {'endpoints': endpoints}}
+        self.unmask_signals()
 
 
 class Manager(obmc.dbuslib.bindings.DbusObjectManager):