Add a default draining START expression for drain command (#7855)

imported from gt7855

When the condor_drain tool is run without specifying the -start argument, the default behavior is is to set Requirements = False and START = undefined. This can be confusing when you use the condor_q -analyze command, and not necessarily what is desired.
There are some machines in CHTC that we would like to have backfill while draining, and it's a pain for a casual user of condor_drain to have to keep track of that.

It should be possible to configure a DEFAULT_DRAINING_START_EXPR in each startd. This will default to nothing, but what it is set, it will be used as the START expression when it gets a drain command without a -start override.

When the defrag daemon is draining, its DEFRAG_DRAINING_START_EXPR will take precedence because that appears to the Startd as a -start override.