Fixing problem with sshare and part_max_priority

With the changes for supporting per partition job priorities, sshare now returns some errors when trying to load the multifactor plugin as part_max_priority can not be resolved. 

As sshare will not really use that variable we just need to initialize it.

However, I do not feel comfortable with this solution. My first idea was to give such a value during plugin initialization as a parameter, but it turned to affect more code than expected.

Maybe you can see a better solution.