Distinguish resize from offline migrate in scheduler

[joker-at-work]

The scheduler reacts to the scheduler-hint _nova_check_type. To be able to have specific filters/weighers for resize, we set this scheduler-hint to "resize" already.

Since resizes and offline migrations take the same code path and only differ in having a flavor change or not, we used to set "resize" also for offline migrations.

In the future, we want to be able to distinguish between offline migrations and resizes, because one is triggered by customers and the other can only be triggered by admins. Therefore, we introduce another _nova_check_type "migrate". There's also already another _nova_check_type "live_migrate", so "migrate" does not clash there.

To help in finding an offline migration scheduling request in the scheduler, we add the helper function request_is_migrate() in the same spirit as the already existing functions for "resize" and "live_migrate".

Change-Id: Id8915a2eb52025e31604af8dde53343ab980ca3e

[fwiesel]

That means that the filters which now check for resize are not going to apply anymore. I am unsure if that should be the way. At leastresize_same_shard I would suspect we want to still apply, possibly volume_size_same_shard_resize_filter as well, no?

[joker-at-work]

resize_same_shard will get an equivalent filter for migrate_same_shard as we also implement live_migrate_same_shard.

volume_size_same_shard_resize_filter explicitly shouldn't continue to work that way, because resizes are triggered by customers and thus need that filter - offline migrations are triggered by admins, which should not be hindered to move a VM around imho.

[joker-at-work]

FYI: #455 Is depending on this PR to implement the other same_shard weighers.