Make driver and platforms config sections optional (#87)

Current implementation mandates that at least driver and platfroms
sections are present in all molecule.yml scenario configuration files.
But because those two pieces of information can also come from the
base configuration file, it does not feel right to fail when they are
not present in the scenario configuration.

This commit makes those two sections optional. Missing platfroms
section is simply interpreted as an empty list since this does not
change the final outcome at all.

Missing driver section is treated a bit differently because the driver
name is used in a few different places. In order not to break
backward-compatibility, we use the "no_driver" string as driver name if
the driver section is not present or if the driver name is not set in
the scenario config.

Author: tadeboro

README.rst

@@ -42,6 +42,9 @@ markers_, so you can selectively limit which test types to run:
     # Runs scenarios with platform named centos7 and delegated driver:
     $ pytest -m delegated -m centos7
 
+If the molecule scenario does not contain information about the driver, the
+test associated with it gets a ``no_driver`` mark.
+
 Please note that at this moment molecule will run the entire scenario if the
 markers are platforms, this is not *yet* a way to limit which platforms are
 executed inside a specific scenario.

pytest_molecule/__init__.py

@@ -95,6 +95,11 @@ def pytest_configure(config):
             )
             config.option.molecule[driver] = {"available": True}
 
+        config.addinivalue_line(
+            "markers",
+            "no_driver: mark used for scenarios that do not contain driver info",
+        )
+
         config.addinivalue_line(
             "markers", "molecule: mark used by all molecule scenarios"
         )
@@ -151,12 +156,17 @@ def __init__(self, name, parent):
         self.funcargs = {}
         super().__init__(name, parent)
         with open(str(self.fspath), "r") as stream:
-            data = yaml.load(stream, Loader=yaml.SafeLoader)
+            # If the molecule.yml file is empty, YAML loader returns None. To
+            # simplify things down the road, we replace None with an empty
+            # dict.
+            data = yaml.load(stream, Loader=yaml.SafeLoader) or {}
+
             # we add the driver as mark
-            self.molecule_driver = data["driver"]["name"]
+            self.molecule_driver = data.get("driver", {}).get("name", "no_driver")
             self.add_marker(self.molecule_driver)
+
             # we also add platforms as marks
-            for platform in data["platforms"]:
+            for platform in data.get("platforms", []):
                 platform_name = platform["name"]
                 self.config.addinivalue_line(
                     "markers",

tests/roles/no_drivers/molecule/default/molecule.yml

@@ -0,0 +1,3 @@
+---
+platforms:
+  - name: localhost

tests/roles/no_platforms/molecule/default/molecule.yml

@@ -0,0 +1,3 @@
+---
+driver:
+  name: delegated