Previously, three different environment variables were required to
modify ports to be used by demo cluster. I think that's unnecessary
and such flexibility comes at greater cost for developers, instead
better to have only one environment variable to help provide the
flexibility. Hence, removing MASTER_PORT and STANDBY_PORT
variables. Now, only PORT_BASE is used. This becomes the starting port
number and master port number. All other required port numbers for
demo cluster are sequential to it, including standby port, followed by
primaries and mirrors.

Also, very often need to create demo cluster across different GPDB
versions. So, would be helpful to have PORT_BASE default set to not
conflict across different GPDB versions. Hence, using 7000 as this is
GPDB7 branch. For GPDB6 we can have it start at 6000 and GPDB5 at
5000. I think that makes it easy to remember as well and shorter to
type as well compared to 5 digits previously used :-)
Signed-off-by: NAshwin Agrawal <aagrawal@pivotal.io>