Don't use -y automatically on drush commands, fixes #1118, fixes #455

[rfay]

The Problem/Issue/Bug:

We have long defaulted to the drush -y, assuming people were using ddev exec or an exec hook, but it makes people crazy when they use drush inside the container.

How this PR Solves The Problem:

• Set environment variable DDEV_EXEC when "exec" is being executed
• /etc/drush/drushrc.php looks at DDEV_EXEC and only adds the -y option if in exec context.

Manual Testing Instructions:

• Run ddev ssh and try drush sql-drop. It will ask first.
• Reload db
• Run ddev exec drush sql-drop and it will just drop the db with -y.

Automated Testing Overview:

Related Issue Link(s):

OP #1118 and #455

[andrewfrench]

Code changes look good, and the test case (drush commands provided from inside the container vs. via ddev exec) behaved as expected.