Removing migration as it needs to be broken up.

This migration does too many things at once. I am breaking it up into
3 parts:
  1. Add all new fields.
  2. Set all null text and char fields that are going to be altered, to the empty
     string.
  3. Alter fields that have been changed.

Refs #13005
Migrations broken up, data migrations added

The full migration has been broken up into 4 parts.

  1. Add fields to VirtualMachine and VirtualMachineTemplate.
  2. Alter code and timestamp fields on GanetiError, doesn't require a
     data migration.
  3. Convert NULLs to blank strings on all serialized_info fields across
     multiple models.
  4. Migrate serialized_info fields.
  5. Convert NULLS to blank strings for VirtualMachineTemplate fields.
  6. Alter VirtualMachineTemplate fields

Refs #13005