Add migration_prep step/var

Copy pasted -
TODO: Use to lower dns TTL to 60 seconds and perform all backups initially.
Anything that is done pre-system/machine migration should be done in a run once (backups) or toggleable (dns TTL) fashion with this setting
variable "migration_prep" { default = false } ## Not in use atm

This will contain an impromptu list for now

• Lower DNS TTL
• Create backups

Deserves its own issue(s)

• Pre-migration/Final step - redirect offsite indicating maintenance
  • Mainly not sure how to redirect normal traffic but allow systems to do what they need.
    Essentially need a list of all bootstrapping behaviour and allow that to hit the new machine while redirecting any 'non-essential' traffic offsite