main: Add command argument to setup essential apps

- The --setup argument sets up all applications that declare themselves
  as essential.

- This is done synchronously.

- Plinth exits after the setup is complete.

- Plinth fails with an error in case any of the setup tasks fail.  The
  process will be continued on next invocation or access of application
  from UI.

plinth/__main__.py

@@ -33,6 +33,7 @@ from cherrypy.process.plugins import Daemonizer
 from plinth import cfg
 from plinth import module_loader
 from plinth import service
+from plinth import setup
 
 logger = logging.getLogger(__name__)
 
@@ -57,6 +58,9 @@ def parse_arguments():
     parser.add_argument(
         '--no-daemon', action='store_true', default=cfg.no_daemon,
         help='do not start as a daemon')
+    parser.add_argument(
+        '--setup', action='store_true', default=False,
+        help='run setup tasks on all essential modules and exit')
     parser.add_argument(
         '--diagnose', action='store_true', default=False,
         help='run diagnostic tests and exit')
@@ -277,6 +281,18 @@ def configure_django():
     os.chmod(cfg.store_file, stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP)
 
 
+def run_setup_and_exit():
+    """Run setup on all essential modules and exit."""
+    error_code = 0
+    try:
+        setup.setup_all_modules(essential=True)
+    except Exception as exception:
+        logger.error('Error running setup - %s', exception)
+        error_code = 1
+
+    sys.exit(error_code)
+
+
 def run_diagnostics_and_exit():
     """Run diagostics on all modules and exit."""
     module = importlib.import_module('plinth.modules.diagnostics.diagnostics')
@@ -314,6 +330,9 @@ def main():
 
     module_loader.load_modules()
 
+    if arguments.setup:
+        run_setup_and_exit()
+
     if arguments.diagnose:
         run_diagnostics_and_exit()