Refactoring driver interfaces

Added method create_share_form_snapshot to
manila.share.driver.ShareDriver.

Removed methods allocate_container, allocate_container_from_snapshot,
create_export, deallocate_container, remove_export

Implements bp refacto-driver-interfaces

Change-Id: I9f1c26b189f50aedc8e19864a172557700adf849
Merge "Refactoring driver interfaces"