Masakari service overview in masakari

[Carlos Law]

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:

- [ ] This doc is inaccurate in this way: ______
- [ ] This is a doc addition request.
- [X] I have a fix to the document that I can paste below including example: input and output.

input
nova-compute host failure - evacuate all the VMs from failure host to

output
nova-compute host failure - evacuate all the VMs from failure host to healthy host

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-----------------------------------
Release: 9.1.0.dev16 on 2019-08-28 20:19:06
SHA: bca0c955a027a92a304777fd844820c34a1612e2
Source: https://opendev.org/openstack/masakari/src/doc/source/install/overview.rst
URL: https://docs.openstack.org/masakari/latest/install/overview.html

[Radosław Piliszek (yoctozepto)]

Well, "evacuation" already conveys the information it will try to find a "better" place for them. Though Masakari cannot guarantee that the new host will be "healthy". It's just up to Nova to reschedule. And Nova only says it allows to "evacuate".