Fix cron validation to reject trailing garbage in fields

[sidtuladhar]

The regex pattern in FieldParser used .match() semantics without an end anchor, allowing invalid expressions like "5kevin" or "*/10abc" to pass validation silently. Adding a $ anchor ensures the entire field must match the pattern.

Fixes TRON-1761

[nemacysts]

@sidtuladhar i've added some of the CIB folks as reviewers as well :)

[nemacysts]

that said, i'm not 100% sure that this will prevent the issue we saw: 10 14 15-21 * 5 is technically a valid cron schedule, it's just not something that tron handles (i assume 'cause it was a pain to implement when this code was initially written)

could you test running what paasta validate runs (https://github.com/Yelp/paasta/blob/master/paasta_tools/tron_tools.py#L1353) on a yaml file that looks like the one that broke sync-deploy using this change?

[sidtuladhar]

@nemacysts right, it wouldn't prevent that. When i run paasta validate on the yaml that has the broken cron syntax for tron, there are no errors

[sidtuladhar]

this prevents monthday and weekday in the parsing now