ctdb-scripts: New internal variable CTDB_SCRIPT_VARDIR

ctdb-scripts: New internal variable CTDB_SCRIPT_VARDIR

Clearly identifies where all state files from scripts should go and
allows that location to be easily changed.  This patch should not
change any behaviour (outside of eventscript unit tests, where a
clearer location is now used).

CTDB_VARDIR should no longer be overridden.  Continue to set
CTDB_DBDIR and similar to override database location in unit tests.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Jose A. Rivera <jarrpa@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>