Extend /resize command with disk_size

itacloud-2565
Move disk_size of /resize command from celery tasks to the REST, add tests

itacloud-2565
Merge branch 'develop' into feature/itacloud-2565
Extract disk resizing and flavor changing to separate functions

itacloud-2565
Add case for unexisted flavor uuid

itacloud-2565
Merge branch 'develop' into feature/itacloud-2565

Conflicts:
	nodeconductor/iaas/tasks.py
	nodeconductor/iaas/views.py
Change reize_disk function

itacloud-2565
Remove unnecessary request to the database

nc-2565
Fix new_flavor cloud checking in change_flavor function

itacloud-2565
Merge branch 'feature/itacloud-2565' into 'develop'

Feature/itacloud 2565