Pin virtualenv to 1.10.1 to work around pip bug

If 1.11 is used with --system-site-packages pip won't be installed in
the virtualenv, leading to an activated virtualenv not using pip and
thus not installing anything into the virtualenv.

Change-Id: Ic9907438b17ca9a32fbeccc4ca806b8b43f41cd1
Partial-Bug: #1266513