Only complain about config.yaml version on start if necessary, fixes #957

[rfay]

The Problem/Issue/Bug:

OP #957 - On update of ddev people were getting plowed under with warnings about every project they had running, and there was no context about with config.yaml needed to be updated.

How this PR Solves The Problem:

• Only warn on ddev start, not every single place a config is read
• Add the full path to the warning so people can understand which config.yaml is in play.

Manual Testing Instructions:

See OP #957 - Use ddev list and ddev start

Related Issue Link(s):

OP #957

[andrewfrench]

ddev list no longer prints out an imposing list of config version warnings, but ddev start will print a warning and specify the full path to the relevant config file.

Moving the version check to app.Start() makes sense to me, approved.