ZTS: Re-enable MMP tests

Commit 7fab6361 inadvertently disabled the MMP test cases by creating
and not removing an /etc/hostid file in the new zpool_split_props test
case. When the file exists the ZTS skips the entire MMP test group
rather than modify what may be a system which is already configured.
Update the test case to remove the file.

Additionally, because the MMP tests were disabled a regression slipped
in as part of commit 9eb7b46ed0. Fix it.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7514