Enclose all CDEF, VDEF, AREA, LINE1, GPRINT inside of single-quotes
(avoids shell quoting issues).  Also adjust other instances of
rrd_gen_graph() string building to use single-quotes to wrap around
filenames that will be passed to /usr/bin/rrdtool.  This avoids a
situation where special characters inside the string literal are
interpreted by the shell during the execution of the rrdtool statement.