cc_salt_minion freebsd fix for rc.conf (SC-782)

[holmanb]

cc_salt_minion freebsd fix for rc.conf

This fixes a bug that prevents the salt module from enabling the salt minion in rc.conf.

For more details:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254339

[igalic]

This looks good to me!

[blackboxsw]

LGTM.
Normally my mantra would have been more unittests or integration tests.... but

• no facility currently in upstream integration tests yet to drive BSD runs
• unittest coverage already covers bsd_utils.set_rc_config_value
• this changeset is minor enough that given our pylint coverage to assert we haven't bungled the import and existing unittests coverage of bsd_utils/set_rc_config_value, we don't really gain much by instrumenting unittests for this path.