VNX: Allow set migrate rate when migrating volumes

VNX: Allow set migrate rate when migrating volumes

VNX driver is leveraging LUN migration functionality when
clone/migrate/retype volume. The migration rate is set to 'high'
by hard code.

This patch will allow admin to control the migration rate by
adding volume metadata before triggering any operations involving
LUN migration.

The required metadata key is 'migrate_rate', available metadata
values are 'high', 'asap', 'low' and 'medium'.

DocImpact
Change-Id: Ie9a875dd63dd60351e46a66c99977d9b6fd23244
Closes-Bug: 1529553