[Repair] Issue warning for improper repair schedule

[vreniers]

HEAD 9dc2b7e

Description

One of the key capabilities of Scylla Manager is to schedule and conduct repair. It would be prudent to also issue a warning when a user has an inappropriate schedule on which to run the repair.

For example:

• A user schedules a weekly repair, however one of the tables has a gc_grace_seconds window of 2 days.

The above should result in a warning to the user.

(cc @vladzcloudius)