ddev exec command should allow tty interactions

[rfay]

The Problem/Issue/Bug:

People have long been frustrated by the inability to interact with the console using ddev exec. But we long since have been able to do that, but didn't add it to the ddev exec command.

How this PR Solves The Problem:

Allow interaction when doing ddev exec. Kind of makes it identical to ddev ssh, but no direct interaction with the container.

Manual Testing Instructions:

ddev exec mysql
ddev exec vi index.php

Automated Testing Overview:

Related Issue Link(s):

#1023 will be adding bash interpretation to ddev exec
#1162 will eventually make ddev exec handle stdout/stderr and exit values correctly

Release/Deployment notes:

This will be an important announcement, and could conceivably affect some people's workflow.