steps, nmci: add steps and function to update keyfiles
The newly created nmci.misc.keyfile_update() function and accompanied steps update the given configfile/keyfile with the configuration given as a string with the same format.
Note: the function and steps do not handle backups or cleanups in any way, they should only be used for ephemeral files or for permanent changes to the system.
Limitations: there's no way to completely remove the configuration key or section with this straightforward implementation, the maximum possible is to set an empty value of the given string
@runtests:pass,doc_firewalld_zones_via_keyfile
Edited by David Jaša