Replace duplicated code snippet with calls to helper functions

Replace duplicated code snippet with calls to helper functions

Reduce code duplication by replacing a code snippet repeated throughout
system tests using "trusted-keys" and/or "managed-keys" configuration
sections with calls to keyfile_to_{managed,trusted}_keys() helper
functions.

(cherry picked from commit dce66f763537450fa78045c679c53fd0ba5cbc5d)