ignore jinja lines in pillar/id/* in CI tests
    
    get_roles.py and test_custom_grains.py fail with the added {% if ... %}
    conditions in pillar/id/*
    
    Add a read_file_skip_jinja() function that reads files and drops jinja
    lines (starting with '{%'), and use this function in get_roles.py and
    test_custom_grains.py instead of f.read().
    
    Filtering out jinja is not 100% technically correct, but good enough for
    the CI tests.
    
    Note that read_file_skip_jinja() is only used for reading pillar/id/*