ipadelegation: Fix idempotence issues due to capitalization.

This patch force processing of permission, attribute and group
attributes in lower case, to match behavior of IPA CLI, transforming
all of them into lowercase characters.

The new behavior fixes idempotence issues when mixing different
capitalization in different tasks for the same attribute.

A new test playbook is avaiable at:

    tests/delegation/test_delegation_member_case_insensitive.yml