Don't stop volume service for failed re-export operations

Commit b71570 "Set vol driver initialized before deleting volumes"
changed the manager behavior to call set_initialized() before
attempting to re-export volumes.

Drivers should not be considered initialized before re-export has
succeeded.  Otherwise a failure to export causes the volume
service to stop, when that failure should be handled like any other
failure to initialize.

Closes-Bug: 1257049
Related-Bug: 1232177
Change-Id: Ic6bc89ef3f15dbbc971fdd8c91117cccb5c2801b
Bump to sqlalchemy-migrate 0.8.2

This allows us to drop some unsightly sqlalchemy-migrate 0.7.3
patching.

The related requirements change is here:

  Icde6248ca102884323d7912bab0f4a716aa294ec

Change-Id: I5f80dfc07f8256716d025002fc1a1f47e58b2142
Use model_query() in db.*****_destroy

Use model_query() instead of session.query in db.volume_destroy,
db.volume_type_destroy, db.transfer_destroy and db.snapshot_destroy as
other methods use.
model_query function is the query helper which is used in most part of
cinder/db/sqlalchemy/api.py.

Change-Id: I97be252df9d40bb97231e3341d5cedec26af2d93
Closes-Bug: #1256483
Add more logging to migrate_volume_completion

Add a debug message on entering the function, and an info message if
it was called to clean up an error. This is useful because Nova copies
the data of in-use volumes, and if it fails there will only be a
message in the Nova logs.

Change-Id: I546d0bae8639072727ebe23394885d40255ba1d4
Merge "Use model_query() in db.*****_destroy"
Merge "Add more logging to migrate_volume_completion"
Merge "Don't stop volume service for failed re-export operations"
Merge "Bump to sqlalchemy-migrate 0.8.2"