syncthing_folder: Accept path to the configuration file

Specifying `config_file` overrides the default path relative to
the current user.

This is handy when the executing user is a different user.

library/storage/syncthing/syncthing_folder.py

@@ -62,6 +62,12 @@ options:
             - API key to use for authentication with host.
               If not provided, will try to auto-configure from filesystem.
         required: false
+    config_file:
+        description:
+            - Path to the Syncthing configuration file for automatic
+              discovery (`api_key`). Note that the running user needs read
+              access to the file.
+        required: false
     timeout:
         description:
             - The socket level timeout in seconds
@@ -116,7 +122,10 @@ def make_headers(host, api_key):
 
 def get_key_from_filesystem(module):
     try:
-        stconfigfile = os.path.expandvars(DEFAULT_ST_CONFIG_LOCATION)
+        if module.params['config_file']:
+            stconfigfile = module.params['config_file']
+        else:
+            stconfigfile = os.path.expandvars(DEFAULT_ST_CONFIG_LOCATION)
         stconfig = parse(stconfigfile)
         root = stconfig.getroot()
         gui = root.find('gui')
@@ -228,6 +237,7 @@ def run_module():
             choices=['sendreceive', 'sendonly', 'receiveonly']),
         host=dict(type='str', default='http://127.0.0.1:8384'),
         api_key=dict(type='str', required=False, no_log=True),
+        config_file=dict(type='path', required=False),
         timeout=dict(type='int', default=30),
         state=dict(type='str', default='present',
                    choices=['absent', 'present', 'pause']),