Add separate lines for each external_links, fixes #1235

[rfay]

The Problem/Issue/Bug:

OP #1235: With a single hostname (the default situation, like "somesite.ddev.local", the name was resolved from within the web container as being the ddev-router.

However, as soon as you added additional_hostnames or additional_fqdns, this feature disappeared

How this PR Solves The Problem:

The docker-compose.yaml was being incorrectly generated. A separate entry in external_links: was required for each of those entries.

Manual Testing Instructions:

• Create a project with additional_hostnames and additional_fqdns.
• ddev ssh
• You should be able to curl http://hostname1.ddev.local successfully (it should be going to ddev-router)
• Verify that ping hostname1.ddev.local actually resolves to ddev-router's IP address.

Automated Testing Overview:

TestInternalAndExternalAccessToURL() was updated to do lots of hits both inside and outside.

Related Issue Link(s):

OP #1235

Release/Deployment notes:

03d065b fixes a bug where ComposeWithStreams() did not return the error it encountered, which caused app.Exec() to misbehave. It's possible this will affect some other behavior.

[andrewfrench]

Approving with the following configuration and test:

• update additional_hostnames to include myhostname
• update additional_fqdns to include myhostname.com
• ddev start
• ddev ssh
• curl myhostname.ddev.local => index.php response
• curl myhostname.com => index.php response
• ping myhostname.ddev.local => resolves and receives response
• pint myhostname.com => resolves and receives response