wsrep_append_foreign_key() and wsrep_append_key() used to take a boolean
argument denoting whether the relevant certification key type is shared
(assuming it is exclusive if the argument is false). Change that
argument to the enum wsrep_key_type from wsrep_api.h, so that eventually
other types can also be passed (like WSREP_KEY_SEMI).

This is a non-functional change.

(cherry picked from commit 360bf36dbb9378b36ef57921c725a9505e19e0d9)