High: ocf-shellfuncs: Pass exit string prefix as env variable and use formatting for printf"

Previously we did some autoconf magic to set the exit string prefix.
Now that prefix is passed into the OCF scripts using the
OCF_EXIT_STRING_PREFIX environment variable. This will reduce complexity
and improve reliability as it eliminates the build dependency requirement.

Also, the exit string is now formatted by printf in a way that
will allow things like '%' characters to exist in the reason string.

Thanks to Lars Ellenberg for the helpful suggestions and code review!