add a -reason to the condor_drain command
condor_drain should have a `-reason` argument like condor_hold does. The reason It should be passed on to the STARTD and advertised in the slot while it is in the draining or drained state. We will use `DrainReason` as the attribute name. `DrainReason` should default to "by <username>" when it is not given as an argument to condor_drain. the condor_defrag daemon should always set the reason to "Defrag".
In a separate ticket, we will change the condor_defrag daemon to use the reason attribute to distinguish between drain a command that it initiated and those initiated by an admin.