Added fix for path-based validators

We support using lxdock from subfolders but Voluptuous doesn't play well in this context, especially for configuration values corresponding
to paths relative to the directory containing the LXDock configuration file. For now we'll just chdir to the homedir (containing the LXDock
file) in order to perform schema validations.

lxdock/conf/config.py

@@ -60,14 +60,20 @@ def from_base_dir(cls, base_dir='.'):
         # Loads the YML.
         config.load()
 
-        # Validates the content of the configuration.
+        # Validates the content of the configuration. We chdir into the home directory of the
+        # project in order to ensure that IsFile/IsDir validators keep working properly if the
+        # config is initialized from a subfolder of the project.
+        cwd = os.getcwd()
+        os.chdir(config.homedir)
         try:
             schema(config._dict)
         except Invalid as e:
             # Formats the voluptuous error
             path = ' @ %s' % '.'.join(map(str, e.path)) if e.path else ''
             msg = 'The LXDock file is invalid because: {0}'.format(e.msg + path)
             raise ConfigFileValidationError(msg)
+        finally:
+            os.chdir(cwd)
 
         # Loads the containers.
         config.load_containers()