Add capm3_release_branch as an env var

This capm3__release_branch environment variable will be passed to pipeline and to
the dev-env tests below in the hierarchy to determine for which CAPM3
and IPAM branch the test should be running.

Change-Id: I6845050fcc0b0a7f16481dfe84ddb4551bb5fb4a
diff --git a/jjb/metal3/job_bml_main_integration_tests.yml b/jjb/metal3/job_bml_main_integration_tests.yml
index aa2b39c..e532ded 100644
--- a/jjb/metal3/job_bml_main_integration_tests.yml
+++ b/jjb/metal3/job_bml_main_integration_tests.yml
@@ -59,6 +59,10 @@
         default: '{image_os}'
         description: 'Distribution to use for the target host as well as source and target cluster. Can be ubuntu or centos'
     - string:
+        name: capm3_release_branch
+        default: 'main'
+        description: 'The target branch of CAPM3 to be used for this test'
+    - string:
         name: CAPI_VERSION
         default: 'v1beta1'
         description: 'Cluster API version.'