Limit number of pacemaker-cluster-member nodes to 31

According to the following, it is not supported to have more than 32
nodes in a cluster:
https://www.suse.com/documentation/sle_ha/book_sleha/data/sec_ha_requirements_hw.html

Since we have one founder, that means 31 non-founders.
Merge pull request #42 from vuntz/limit-members

Limit number of pacemaker-cluster-member nodes to 31