add a new tunable "CheckNodesFile" that when set to 0 will disable the

add a new tunable "CheckNodesFile" that when set to 0   will disable the
check in the recovery daemon that all nodes are using the same
/etc/ctdb/nodes file.

Also add some more missing checks that the pnn used is a valid pnn
before using it to dereferencing the ctdb->nodes array

This is useful since it allows us to add more physical nodes to a an
existing cluster without having to bring down the entire cluster.

The to add an additional node to an existing cluster would then be
1, on all nodes set CheckNodesFile=0 using 'ctdb setvar'
2, on all nodes add CTDB_SET_CheckNodesFile=0 to /etc/sysconfig/ctdb
For each each node, one at a time :
3, use 'ctdb disable' to stop the hosted services
4, service ctdb stop
5, service ctdb start
Once all nodes have been restarted
6, on all nodes remove CTDB_SET_CheckNodesFile=0 from
/etc/sysconfig/ctdb
7, on all nodes set CheckNodesFile=0 using 'ctdb setvar'

8, configure and start up the new node

During this procedure, only one node at a time was brought
down/restarted and was so only for a short period.