tox: Add py2-rpmdeps testenv

This avoids grabbing requirements from PyPI. Use this testenv if you
want to only test with globally-available (read: RPM) requirements.

Signed-off-by: Zack Cerza <zack@redhat.com>

tox.ini

@@ -59,3 +59,16 @@ commands =
         tests/test_plugins/test_save_failed_tree_cli.py
     {envbindir}/coverage2 report
     {envbindir}/coverage2 html -d {toxinidir}/htmlcov/py3
+
+# This is identical to the py2 testenv, but without attempting to install
+# dependencies from requirements.txt. In practice this will mean RPM-based
+# dependencies must be installed.
+[testenv:py2-rpmdeps]
+deps =
+    -r{toxinidir}/test-requirements.txt
+setenv =
+    {[testenv:py2]setenv}
+commands_pre =
+    {[testenv:py2]commands_pre}
+commands =
+    {[testenv:py2]commands}