[wishlist] Add config option for live-migration force-completion strategy

[Drew Freiberger]

In the openstack documentation for nova live migration, three different strategies for migration completion are provided. The charm should expose this as a variable with the following options:

- pause
- auto-converge
- post-copy

The standard live-migration force completion is to pause the instance while the remaining data is copied, but this can cause havoc for some workloads that rely on time being relatively linear and may be problematic for customers working with regulatory compliance for date-time tracking of transactions.

While this is not a common need, some sites have very large memory segment workloads that require zero downtime and the ability to alter this strategy via standard charm config rather than using config-flags would be quite useful.

https://docs.openstack.org/nova/pike/admin/live-migration-usage.html#addressing-migration-timeouts

[Drew Freiberger (afreiberger)]

Looks like this is close to the request of lp#1799916 and is part of the charm. the options are:
  live_migration_permit_auto_converge
  live_migration_permit_post_copy

and can be set to true.

These should be mutually exclusive options, and the charm should block if both = True.