Add function for Flask app to read config from a directory.

The application now checks for an environment variable,
OVERRIDE_CONFIG_DIRECTORY. If that value is set, it loops all the files
in the specified directory and checks if their names correspond to known
configuration settings. The contents of any matching files are read and
set as the new configuration value for that setting. This will allow us
to read mounted Azure Key Vault items as config values.

This also moves the functionality for applying environment variables to
the config into an analogous function.

atst/app.py

@@ -200,23 +200,21 @@ def make_config(direct_config=None):
     ENV_CONFIG_FILENAME = os.path.join(
         os.path.dirname(__file__), "../config/", "{}.ini".format(ENV.lower())
     )
-    OVERRIDE_CONFIG_FILENAME = os.getenv("OVERRIDE_CONFIG_FULLPATH")
+    OVERRIDE_CONFIG_DIRECTORY = os.getenv("OVERRIDE_CONFIG_DIRECTORY")
 
     config = ConfigParser(allow_no_value=True)
     config.optionxform = str
 
     config_files = [BASE_CONFIG_FILENAME, ENV_CONFIG_FILENAME]
-    if OVERRIDE_CONFIG_FILENAME:
-        config_files.append(OVERRIDE_CONFIG_FILENAME)
 
     # ENV_CONFIG will override values in BASE_CONFIG.
     config.read(config_files)
 
+    if OVERRIDE_CONFIG_DIRECTORY:
+        apply_config_from_directory(OVERRIDE_CONFIG_DIRECTORY, config)
+
     # Check for ENV variables as a final source of overrides
-    for confsetting in config.options("default"):
-        env_override = os.getenv(confsetting.upper())
-        if env_override:
-            config.set("default", confsetting, env_override)
+    apply_config_from_environment(config)
 
     # override if a dictionary of options has been given
     if direct_config:
@@ -244,6 +242,36 @@ def make_config(direct_config=None):
     return map_config(config)
 
 
+def apply_config_from_directory(config_dir, config, section="default"):
+    """
+    Loop files in a directory, check if the names correspond to
+    known config values, and apply the file contents as the value
+    for that setting if they do.
+    """
+    for confsetting in os.listdir(config_dir):
+        if confsetting in config.options(section):
+            full_path = os.path.join(config_dir, confsetting)
+            with open(full_path, "r") as conf_file:
+                config.set(section, confsetting, conf_file.read().strip())
+
+    return config
+
+
+def apply_config_from_environment(config, section="default"):
+    """
+    Loops all the configuration settins in a given section of a
+    config object and checks whether those settings also exist as
+    environment variables. If so, it applies the environment
+    variables value as the new configuration setting value.
+    """
+    for confsetting in config.options(section):
+        env_override = os.getenv(confsetting.upper())
+        if env_override:
+            config.set(section, confsetting, env_override)
+
+    return config
+
+
 def make_redis(app, config):
     r = redis.Redis.from_url(config["REDIS_URI"])
     app.redis = r