Ensure external drivers are installed (#3901)

If someone try installing plugins (say docker) with molecule[docker],
nothing will get installed as it should be molecule-plugins or
molecule-plugins[docker].
This leads to a python backtrace similar to the one found in
#3883.
This should be handled properly, so check that the driver is in
api.drivers() before trying to get the value.

Signed-off-by: Arnaud Patard <apatard@hupstream.com>
Co-authored-by: Daniel Ziegenberg <daniel@ziegenberg.at>
Co-authored-by: Sorin Sbarnea <ssbarnea@redhat.com>

src/molecule/config.py

@@ -182,7 +182,12 @@ def driver(self):
         driver_name = self._get_driver_name()
         driver = None
 
-        driver = api.drivers(config=self)[driver_name]
+        api_drivers = api.drivers(config=self)
+        if driver_name not in api_drivers:
+            msg = f"Failed to find driver {driver_name}. Please ensure that the driver is correctly installed."
+            util.sysexit_with_message(msg)
+
+        driver = api_drivers[driver_name]
         driver.name = driver_name
 
         return driver