Add a clear explanation how FLOW_CONTEXT is set, for what host names

[ryzy]

Seem like people are confused why and how the FLOW_CONTEXT is set, for which hostnames. They often use 'neos.test' for local development which switches context to Testing and nothing works (as this context is by default configured to use sqlite and for running functional tests).

Question: do we really need that Testing context set for Nginx? Seems like it's used only for functional tests, which use internal client, not server vhost.

[ryzy]

Testing FLOW_CONTEXT removed as it was never needed.

See http://docs.typo3.org/neos/TYPO3NeosDocumentation/DeveloperGuide/Testing/Behat.html for more info.