Document how to resolve the error "Multiple head revisions are present for given argument 'head'"

[hongbin]

This is the common error encountered by our contributors:

Multiple head revisions are present for given argument 'head'

We need to document how to resolve it.

[Martin Falatic (martinfalatic)]

Can you give more detail on when exactly this comes up?

[hongbin (hongbin034)]

@Martin, this error will come up if
* The Magnum DB is on version A.
* User generated a DB migration locally, which migrate the DB from version A to version B.
* User pull from master that contains another migration from version A to version C.

Then, this error will show up. It means there are multiple "head" (version B and C).

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/514090

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/514090
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=7482e65a2d639a39762e398c91e3059dffc5c250
Submitter: Zuul
Branch: master

commit 7482e65a2d639a39762e398c91e3059dffc5c250
Author: Kirsten G <email address hidden>
Date: Mon Oct 23 13:11:50 2017 -0700

    Doc Fix for Alembic multiple heads error

    Add documentation to fix multiple heads Alembic error. Create file
    contributor/troubleshooting.rst and update contributor/index.rst.

    Closes-Bug: 1548371

    Change-Id: I957bf1c7529f9409d8026f43d5fa8e04b9278d61

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/magnum 6.0.0 release.