(PUP-1586) allow managing existing cronjobs without caring for command

When the catalog contains a cron resource that does not specify a value
for the command property, the transaction would fail with the cryptic
error message 'no command, somehow' under certain conditions.

This error is spurious. Generally, it's quite allowable to manage a subset
of cron properties that does not include the command. Fixed by removing
the devfail invocation. The command property is now handled just like
the 'special' property.