Support for cron expressions

[nestoru]

It would be convenient to add support for cron expressions. To illustrate why let us consider the need to run a backup every hour for 7 years (not atypical for some Financial Services needs). We are forced to tag each instance 24 times:

scheduler:ebs-snapshot:prod:00	0000;2555;utc;all
scheduler:ebs-snapshot:prod:01	0100;2555;utc;all
scheduler:ebs-snapshot:prod:02	0200;2555;utc;all
scheduler:ebs-snapshot:prod:03	0300;2555;utc;all
scheduler:ebs-snapshot:prod:04	0400;2555;utc;all
scheduler:ebs-snapshot:prod:05	0500;2555;utc;all
scheduler:ebs-snapshot:prod:06	0600;2555;utc;all
scheduler:ebs-snapshot:prod:07	0700;2555;utc;all
scheduler:ebs-snapshot:prod:08	0800;2555;utc;all
scheduler:ebs-snapshot:prod:09	0900;2555;utc;all
scheduler:ebs-snapshot:prod:10	1000;2555;utc;all
scheduler:ebs-snapshot:prod:11	1100;2555;utc;all
scheduler:ebs-snapshot:prod:12	1200;2555;utc;all
scheduler:ebs-snapshot:prod:13	1300;2555;utc;all
scheduler:ebs-snapshot:prod:14	1400;2555;utc;all
scheduler:ebs-snapshot:prod:15	1500;2555;utc;all
scheduler:ebs-snapshot:prod:16	1600;2555;utc;all
scheduler:ebs-snapshot:prod:17	1700;2555;utc;all
scheduler:ebs-snapshot:prod:18	1800;2555;utc;all
scheduler:ebs-snapshot:prod:19	1900;2555;utc;all
scheduler:ebs-snapshot:prod:20	2000;2555;utc;all
scheduler:ebs-snapshot:prod:21	2100;2555;utc;all
scheduler:ebs-snapshot:prod:22	2200;2555;utc;all
scheduler:ebs-snapshot:prod:23	2300;2555;utc;all

We could tag just once. Considering the typical linux cron "m h dom mon dow":

scheduler:ebs-snapshot:prod	00 23 * * *;2555;utc

Note that the last "all" is not longer necessary. If there are spaces in the first value token it would mean it is a cron and only three tokens in total would be expected.