remove downgrade option from tacker-db-manage

[Kanagaraj Manickam]

From Kilo version, OpenStack decided to not to support downgrade of db.
In tacker, it is not followed on some db version scripts.

This bug is filed to clean them.

Also tacker-db-manage command is updated to remove the option downgrade.

After the fix, the command would look like this
usage: cli [-h] [--database-connection DATABASE_CONNECTION]
           [--database-engine DATABASE_ENGINE] [--config-dir DIR]
           [--config-file PATH]
           {current,history,branches,check_migration,upgrade,stamp,revision,purge_deleted}

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/439718

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/439718
Committed: https://git.openstack.org/cgit/openstack/tacker/commit/?id=3ca19f068e939bd2f49e7462056495a13f58b043
Submitter: Jenkins
Branch: master

commit 3ca19f068e939bd2f49e7462056495a13f58b043
Author: Kanagaraj Manickam <email address hidden>
Date: Wed Mar 1 12:50:50 2017 +0530

    Removes downgrade from db migration scripts

    downgrade is not to be supported from kilo release
    onwards and is good to clean up the code.

    Closes-bug: #1668887

    Change-Id: I4b3f264d8691b74d602df466752eccbea533c209

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/tacker 0.8.0 release.