Remove duplicate code from nova.db.sqlalchemy.utils

There is a lot of code in nova.db.sqlalchemy.utils which is also in
oslo-incubator's openstack.common.db.sqlalchemy.utils module so this
patch goes through and cleans up Nova.

Notable differences:

1. modify_indexes was nova-only but no longer used after the migration
   compaction work, so it's just removed here. It also didn't really
   work for mysql in cases where the index name was too long so it
   could be table-specific and therefore unusable in some mysql cases.
   See commit 029ebab for history.
2. create_shadow_table was not in oslo-incubator and it's also not
   currently used in nova after the migration compaction, however,
   I leave it here in case future migrations in Nova need to use it.
   This patch does replace the usage of _get_not_supported_column in
   create_shadow_table to use the same method from oslo's DB utils.
3. DeleteFromSelect is still used within nova.db.api per
   commit b36826e so we could move it to oslo-incubator but this
   patch leaves it for now since it's only used in Nova.
4. InsertFromSelect was introduced with commit 2e403b2 but now there
   is a copy in oslo-incubator so we can remove our usage in nova.
   However, oslo doesn't have the unit test that nova does, so the
   unit test has been moved to oslo with change I457acf33.
5. Oslo has is_backend_avail and get_connect_string methods so use
   those in test_migrations.py while doing this update.

Closes-Bug: #1280055

Change-Id: Iefa5b4311f1fe1a5da31cf527521c393f2face7c
Return to using network_api directly for migrations

Previously, we added indirection APIs in conductor to wrap network
methods that still had direct-to-database calls that compute
manager couldn't make. Now that those are cleaned up, we can
remove the use of the indirection layer (and the layer itself from
conductor) and go direct to network_api again.

Related to blueprint compute-manager-objects-juno

Change-Id: Ia76552f49adf9b0d0128f08e3f99953560d8aa64
Merge "Return to using network_api directly for migrations"
Merge "Remove duplicate code from nova.db.sqlalchemy.utils"