Change USE_PYTHON3=True by default

Since Stein, gate jobs have been using bionic nodes so they
are running with python 3.6, so it makes sense to also default
devstack itself to run with python3 by default.

Depends-On: https://review.opendev.org/688731

Change-Id: I52b03caee0ba700da3a15035201ea6cd91baa06b

Author: mriedem

stackrc

@@ -136,7 +136,7 @@ if [[ -r $RC_DIR/.localrc.password ]]; then
 fi
 
 # Control whether Python 3 should be used at all.
-export USE_PYTHON3=$(trueorfalse False USE_PYTHON3)
+export USE_PYTHON3=$(trueorfalse True USE_PYTHON3)
 
 # Explicitly list services not to run under Python 3. See
 # disable_python3_package to edit this variable.