Use a task subclass instead of a functor + task wrapper

Instead of using a functor that activates the scheduler driver
instead incorporate and use a task subclass, this avoids the less
meaningful information that is presented when a functor is used
as well as allows for a more straightforward path for future
subclassing and derivations of work.

Change-Id: Ice66e109968c28f36156edf3dd47a89f94494bb8
Don't use ModelBase.save() inside of transaction

'with session.begin()' makes some operations in one transaction.
session.begin() returns a transaction instance, then does some operations,
and will commit or rollback automatically before leaving the block.
ModelBase.save() always submit a commit, and that is not expected.
When we get a persistent object from database, we just modify the
object inside of block 'with session.begin()' and sqlalchemy will
update it, don't need method session.add() or ModelBase.save().

Closes-Bug: #1224429
Change-Id: I4af58e98b2783d3945d92e57680d58e7ae356a67
Add cinder-manage cmd to update host column

If you deploy Cinder as per the default using the docs,
you can't later decide to add backends using the
multi-backend functionality. The reason is that multi-backend
is implemented by appending a backend-name to the host entry.

So if you convert the config to multi-backend the hostname
for any volumes that you have created are no longer
associated with a valid volume-driver (ie you can no
longer perform operations including delete on these
volumes).

In addition, in the case of using a SAN attached backend if
one redeploys their cinder-volume service to another node
they'll have the same issue.

This patch adds a cinder-manage command that will go through
volumes that are located on a specified host and update
the host column on each of them.

Change-Id: I8758fb8ef3fccbba2a598240c42d625b997e2db6
Closes-Bug: 1320688
Merge "Use a task subclass instead of a functor + task wrapper"
Merge "Add cinder-manage cmd to update host column"
Merge "Don't use ModelBase.save() inside of transaction"