PROD-36050 Add option to skip switching to proposed branch

Ususally we switch to proposed branch during the MCP update process
but when we have released version we don't need that switching

Use --dont-switch-to-proposed pytest option in command line
to disable this switching.

Change-Id: Id1b07ca6651eb3c7c3c97a585cb32efa672eb620
diff --git a/tcp_tests/tests/system/conftest.py b/tcp_tests/tests/system/conftest.py
index 0627ede..2ea36cf 100644
--- a/tcp_tests/tests/system/conftest.py
+++ b/tcp_tests/tests/system/conftest.py
@@ -80,3 +80,11 @@
     # tempest
     'tempest_actions'
 ])
+
+
+def pytest_addoption(parser):
+    parser.addoption("--dont-switch-to-proposed",
+                     action="store_true",
+                     help="Skips switching Jenkins on cluster-under-test to "
+                          "the proposed branch before the applying "
+                          "the MCP updates")