Switch to separate config file version

Use a separately incremented integer to track the configuration file
format version. We start with 2 due to the change that 23c3a951f6
introduced. 1 is declared to be equivalent to the original '0.10'.

The separate versioning has the advantage of being able to increment it
already during the development cycle, and using it with config files
that test/exploit the new format.

Using an integer has the advantages of a) differentiating it clearly
from the now independent kas version and b) simplifying the version
parsing. We can now also remove the string type restriction.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/format-changelog.rst

@@ -0,0 +1,25 @@
+Configuration Format Changes
+============================
+
+Version 1 (Alias '0.10')
+------------------------
+
+Added
+~~~~~
+
+- Include mechanism
+- Version check
+
+
+Version 2
+---------
+
+Changed
+~~~~~~~
+
+- Configuration file versions are now integers
+
+Fixed
+~~~~~
+
+- Including files from repos that are not defined in the current file