Add a variable CTDB_CHECK_SWAP_IS_NOT_USED="yes"

Add a variable CTDB_CHECK_SWAP_IS_NOT_USED="yes"
to control whether or not to check if we are swapping, and produce
useful output into the logfile if we are.

For production systems with dedicated nas-heads we should never swap.
But for developer/test systems we often use smaller nondedicated systems where
we can no longer guarantee that we will not be using swap.