Fix environment variable type (#912)

Author: ebattat

benchmark_runner/main/environment_variables.py

@@ -89,7 +89,7 @@ def __init__(self):
         # Verification only, without running or deleting any resources, default False
         self._environment_variables_dict['verification_only'] = EnvironmentVariables.get_boolean_from_environment('VERIFICATION_ONLY', False)
         # Verification while upgrade, e.g. 4.15.23
-        self._environment_variables_dict['wait_for_upgrade_version'] = EnvironmentVariables.get_boolean_from_environment('WAIT_FOR_UPGRADE_VERSION', '')
+        self._environment_variables_dict['wait_for_upgrade_version'] = EnvironmentVariables.get_env('WAIT_FOR_UPGRADE_VERSION', '')
 
         # default parameter - change only if needed
         # Parameters below related to 'run_workload()'