make pgxc_ctl connect to 'postgres' database when pinging a node

make pgxc_ctl connect to 'postgres' database when pinging a node

pingNode() was not specifying database in the connection string,
so the OS username was used implicitly. When such database did
not exist (e.g. when initializing new nodes, under OS account
different from 'postgres'), this resulted in FATAL failures
in the server log:

    FATAL:  database "user" does not exist

Adding 'database=postgres' to the connection string fixes the
issue. It might be useful to allow using custom database names,
but 'postgres' is pretty much guaranteed to exist.

Bug report by me, patch by Pavan Deolasee.