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/*