MySQL Galera cluster can't initiate first node
Have you tried creating the /var/run/mysqld directory, with rwx permissions for the mysql user? Judging by the error messages, that seems to be a good place to start.

The other error message should probably also be addressed, regarding not writing to syslog and the mysql error log at the same time. I'd suggest inspecting your MySQL config files under /etc/ to see if you can disable the syslog option.

Also, it looks a bit strange that you're using mysql-wsrep-5.6 with MySQL 5.7. Is there a mysql-wsrep-5.7? If not, perhaps you need to use MySQL 5.6.