Remove configuration section about base boxes.

[rosiel]

Furthrer to Islandora-Devops/islandora-playbook#201 (comment)
and in line with Islandora-Devops/islandora-playbook#203

Purpose / why

We no longer default to the islandora/8 box, in fact it's very out of date and should not be used. This also removes documentation that the playbook (in its Vagrant format) works with Centos, since that seems to also have fallen by the wayside

What changes were made?

turfed the mentions of a "quick start" mode and of changing the base box, since it's no longer a real option.

Verification

Do the instructions work?

Interested Parties

Replace this text - Name some folks who may be interested, if documentation related mention @Islandora/documentation, or, if unsure, @Islandora/8-x-committers

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Checklist

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[elizoller]

Seems accurate. Worth noting this PR removes the islandora/8 base box and centos builds from the playbook documentation.