subtests.docker_cli.dockerinspect: Check CamelCase

Fixes: https://github.com/autotest/autotest-docker/issues/260

This patch adds the CamelCase keys check into the
docker_cli/dockerinspect/inspect_all test.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Tip: Normally it's best to keep parameter types uniform and simple (just like return types).  e.g. you could always put the loop-unroll inside ``check_camel_case``.  Also, I like to avoid using recursion in python since it can easily go beyond the limit.  Flattened out, it's easier to debug as well.

However, what you're doing here is simple enough, so no need to fix.