Implement periodic on-line DRBD verification

While working on #41176 (closed) we started using DRBD on-line verification (drbdsetup verify) and discovered that many (all?) our DRBD volumes were out of sync. According to other Ganeti users, this is not a normal situation.

I configured the verify-alg setting in both clusters using gnt-cluster modify --disk-parameters="drbd:net-custom=--verify-alg crc32c"

We should configure an automatic verification to run periodically to ensure we detect if this continues to occur. The DRBD documentation suggests once monthly.