Improvement: unmount NFS exports

Enables optional umounting of NFS exports in
DefaultClusterSetup._remove_nfs_exports.  The was necessary when
sharing /scratch/user over NFS after multilple 'addnode -x -a nodeXXX'
& 'removenode -k' commands.
Improvement: set scheduler polling interval, number of master and node processing slots.

Allows setting the number of processing slots via config file rather
than relying on the number of processors.   Also enables configuration
of the polling interval which can speed up overall queue system when
there are many short duration jobs.
Improvement:  set FILL UP hosts

Sets scheduler policy to fill up one host at a time such that
load_balancer can more easily remove unutilised exec hosts.  Default
SGE policy is to balance jobs across hosts.

adds method _get_number_of_slots, which could be further improved:
* more robust parsing of qconf output
* what happens if there are multiple queues / queue with different name
Avoid duplicate known_host entries for master

A quick fix (maybe raises other problems) for avoiding duplicate
entries for master node in known_hosts for root user on the master node.
Clear error states on SGE

Provide means to clear error states of SGE queue (not currently used
anywhere.
Resubmit jobs on remove node

Resubmit jobs running on the node that is to be removed to other nodes.
Merge branch 'develop' into sge-plugin
Merge branch 'sge-plugin' into develop