Fix indentation in bsdinstall-created wpa_supplicant.conf

r309934 cleaned up some cases in bsdinstall to use heredocs but broke
the indentation of the generated output, because <<- heredocs strip
leading tabs.

PR: 221982
Reviewed by: allanjude, dteske
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13190