Install "ironic" in the unittest environment

This patch is adding an "extra-requirements.txt" file that points to the
Ironic's git repository which will be installed in the virtual
environment where the unittests will run.

Change-Id: I42ccbfde8f3381b9d77f85b3f676c0b455c62851

extra-requirements.txt

@@ -0,0 +1 @@
+git+git://git.openstack.org/openstack/ironic#egg=ironic

tox.ini

@@ -13,7 +13,9 @@ setenv = VIRTUAL_ENV={envdir}
          LANGUAGE=en_US
          LC_ALL=en_US.UTF-8
          TESTS_DIR=./ironic_staging_drivers/tests/unit/
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+    -r{toxinidir}/test-requirements.txt
+    -r{toxinidir}/extra-requirements.txt
 commands = ostestr {posargs}
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY