Install and configure for Ubuntu in magnum

[Olivier De Jonckère]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [x] This doc is inaccurate in this way: "host" section may need to be switch from [api] to [DEFAULT] in "Edit the /etc/magnum/magnum.conf file:"

    In the [api] section, configure the host:
- [ ] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

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Release: 5.0.1.dev132 on 2018-01-15 10:38
SHA: 2a60e56d412c07317fbf3b82de691574d8e7e77b
Source: https://git.openstack.org/cgit/openstack/magnum/tree/doc/source/install/install-ubuntu.rst
URL: https://docs.openstack.org/magnum/latest/install/install-ubuntu.html