config.py: use var to hold defaults.conf string

This way we have all config file names together in same place.

Change-Id: If35739f21f1414efdaba71acbb60661213ac58df
diff --git a/src/osmo_gsm_tester/config.py b/src/osmo_gsm_tester/config.py
index 71e0009..4480338 100644
--- a/src/osmo_gsm_tester/config.py
+++ b/src/osmo_gsm_tester/config.py
@@ -71,6 +71,7 @@
 
 PATHS_CONF = 'paths.conf'
 DEFAULT_SUITES_CONF = 'default-suites.conf'
+DEFAULTS_CONF = 'defaults.conf'
 PATH_STATE_DIR = 'state_dir'
 PATH_SUITES_DIR = 'suites_dir'
 PATH_SCENARIOS_DIR = 'scenarios_dir'
@@ -189,7 +190,7 @@
     return config
 
 def get_defaults(for_kind):
-    defaults = read_config_file('defaults.conf', if_missing_return={})
+    defaults = read_config_file(DEFAULTS_CONF, if_missing_return={})
     return defaults.get(for_kind, {})
 
 class Scenario(log.Origin, dict):