Add constraints-juju-default.txt

This file aims to hold the default version of juju that it's expected
charms to be tested with. It helps to serve as a sane default for
tox.ini

constraints-juju-default.txt

@@ -0,0 +1,9 @@
+# NOTE: this constraints file can be (and will be) consumed by downstream users.
+#
+# Known consumers:
+# * zosci-config: job definitions that declare what juju version (snap channel)
+#   is used in tandem with this constraints file to lockdown python-libjuju
+#   version.
+# * zaza-openstack-tests
+#
+juju>=3.1.0,<3.2.0