Refactor: libcrmcommon: Iterate only once in pcmk__substitute_secrets()

Previously we were iterating over the whole hash table once to filter
out secret parameters into a list, and then iterating over the list to
process the secret parameters. Instead, just iterate over the hash table
and do work if we need to.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>