Add nbroles_to_grains script + add note

Script allows for testing and pipeline minions to work without access to
the roles API. Additionally added a note about this in prepare_minion.py.

Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>

bin/nbroles_to_grains.sh

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+#!/usr/bin/env sh
+# This rewrites top-files to fetch roles from grains instead of our custom roles API. Useful for testing outside of the LibertaCasa infrastructure, but not recommended for production.
+
+sed -i "s/salt\['nbroles.get'\](id)/grains\['roles'\]/" */top.sls

bin/prepare_minion.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# Prepares a minion for local testing of Salt code in this repository. Requires roles to be written as grains, as development and pipeline containers generally do not have access to our roles API. Consider nbroles_to_grains.sh first if this is the case.
 
 import roles
 import os

pillar/id/README.md

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+This directory can be used for machine-specific pillar data.

pillar/role/README.md

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+This directory is used for role-wide pillar data.