config: Do not require all rule options to be set

Before, it was required to specify all the options when customizing a
rule. For instance, one could use `empty-lines: enable` or `empty-lines:
{max: 1, max-start: 2, max-end: 2}`, but not just `empty-lines: {max:
1}` (it would fail with *invalid config: missing option "max-start" for
rule "empty-lines"*).

This was a minor problem for users, but it prevented the addition of new
options to existing rules, see [1] for an example. If a new option was
added, updating yamllint for all users that customize the rule would
produce a crash (*invalid config: missing option ...*).

To avoid that, let's embed default values inside the rules themselves,
instead of keeping them in `conf/default.yaml`.

This refactor should not have any impact on existing projects. I've
manually checked that it did not change the output of tests, on
different projects:
- ansible/ansible: `test/runner/ansible-test sanity --python 3.7 --test yamllint`
- ansible/molecule: `yamllint -s test/ molecule/`
- Neo23x0/sigma: `make test-yaml`
- markstory/lint-review: `yamllint .`

[1]: https://github.com/adrienverge/yamllint/pull/151

yamllint/rules/braces.py

@@ -101,6 +101,10 @@ CONF = {'min-spaces-inside': int,
         'max-spaces-inside': int,
         'min-spaces-inside-empty': int,
         'max-spaces-inside-empty': int}
+DEFAULT = {'min-spaces-inside': 0,
+           'max-spaces-inside': 0,
+           'min-spaces-inside-empty': -1,
+           'max-spaces-inside-empty': -1}
 
 
 def check(conf, token, prev, next, nextnext, context):