Only run post-start hook when the new container is actually created, fixes #1115

[rfay]

The Problem/Issue/Bug:

People who put post-start hooks in really normally want them to run only when the container is created or recreated. A common example is installing an extra Debian package; it only needs to be done when the container is recreated. A start that doesn't recreate doesn't need to run the hook. OP #1115

How this PR Solves The Problem:

• Touch /var/tmp/ddev_started.txt on startup; check to see if it's been touched if we're going to do a hook.
• No action is taken if there are no post-start hooks.

Manual Testing Instructions:

• Create a post-start hook, maybe "bash -c echo hello there"
• ddev start. You should see it fire
• ddev start again, you should not see it fire
• Change the php type and ddev start, container should be recreated and you should see it fire.

Automated Testing Overview:

• Added test for this behavior in TestDdevStart
• Also changed TestDdevStart to only run one CMS, as behavior across all CMSs is the same.

Related Issue Link(s):

OP #1115

Release/Deployment notes:

This does in fact change the behavior of the post-start hook, so needs to be announced. Most people will like it, it's probably more what's expected.

[andrewfrench]

Pretty clever signalling technique - looks good to me:

hooks:
  post-start:
    - exec: bash -c 'echo hello there'

$ ddev start
// post-start hook runs

$ ddev start
// post start hook does not run

$ ddev config --php-version 7.3
$ ddev start
// post-start hook runs

$ ddev start
// post-start hook does not run