Kilo changes for Horizon

Overview
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This blueprint collects all the changes that are necessary to make the Horizon playbook ready for Kilo.
This is meant to supplement and detail the Horizon tasks necessary to fulfill the master blueprint for supporting the Kilo release: https://blueprints.launchpad.net/openstack-ansible/+spec/master-kilofication

Problem Description
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The configuration files need to be updated for Kilo.

Proposed Change
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After an analysis of the changes between Juno and Kilo, we (sigmavirus24, stevelle & miguelgrinberg) have come up with the following list of changes, all in the horizon_local_settings.py.j2 template:

  * Option MAX_DROPDOWN_ITEMS=30 needs to be added.
  * The comment that enumerates the allowed console types needs to include the new SERIAL type.
  * The commented out HORIZON_CONFIG["disable_password_reveal"]=False option needs to be added.
  * The commented out ENFORCE_PASSWORD_CHECK=False option needs to be added.
  * The commented out AUTHENTICATION_URLS option needs to be added.
  * The OPENSTACK_TOKEN_HASH_ALGORITHM=md5 option should probably be changed to use SHA256 (same change needs to be done on the Keystone side). [Note: we are sticking to Horizon defaults for now, so this is out]
  * Do we need the DATABASES and SESSION_ENGINE options? The official config file does not have them. [Answer is yes, we do need these as they configure the server-side session storage]

Playbook Impact
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Minor impact localized to the local_settings.py template file.

Alternatives
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None.

Security Impact
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None. Switching token hash algorighm from the MD5 default to SHA256 will improve security.

Performance Impact
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None.

End User Impact
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No user impacting changes.

Deployer Impact
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No change.

Developer Impact
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No change.

Dependencies
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If the MD5 token hashing is changed to use SHA256, then Keystone has to be changed in the same way.

Documentation Impact
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No changes to docs.

References
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Kilo version of the local_settings.py.example:
https://github.com/openstack/horizon/blob/master/openstack_dashboard/local/local_settings.py.example