Return '' for identify state on read ipmi

For exploring the api, a write-only attribute presents difficulty.  Address this by allowing
plugins to return a '' value and have ipmi plugin return that value to enable things
like the web api explorer.

confluent_server/confluent/messages.py

@@ -452,6 +452,7 @@ class InputBootDevice(BootDevice):
 
 class IdentifyState(ConfluentChoiceMessage):
     valid_values = set([
+        '',  # allowed for output to indicate write-only support
         'on',
         'off',
     ])

confluent_server/confluent/plugins/hardwaremanagement/ipmi.py

@@ -341,6 +341,10 @@ class IpmiHandler(object):
             self.ipmicmd.set_identify(on=identifystate)
             return msg.IdentifyState(
                 node=self.node, state=self.inputdata.inputbynode[self.node])
+        elif 'read' == self.op:
+            # ipmi has identify as read-only for now
+            return msg.IdentifyState(node=self.node, state='')
+
     def power(self):
         if 'read' == self.op:
             power = self.ipmicmd.get_power()