Symlink tox into the exec path for release jobs

Since release tooling tends to call tox from shell scripts, and we
now install tox into a non-default location (a virtualenv outside
the typical execution path), pass a parameter telling the ensure-tox
role to symlink it into /usr/local/bin.

Change-Id: I2fa5dfdce8d90ef9bf42b3332c135e83c4ca82fc
Depends-On: https://review.opendev.org/736330

playbooks/release/pre.yaml

@@ -6,6 +6,7 @@
     - role: bindep
       bindep_profile: test
       bindep_dir: src/opendev.org/openstack/releases
-    - ensure-tox
+    - role: ensure-tox
+      ensure_global_symlinks: true
     - ensure-virtualenv
     - revoke-sudo