Disable utility mode in singlenode deployment

[wfnuser]

Utilty mode is not necessary in singlenode deployment.
But if we disable directly, many of the isolaiton tests will be broken. "1U:" grammar require connection to be set in utility mode.

Can be fixed by this.

void
assign_gp_role(const char *newval, void *extra)
{
	/*
	 * Since postgres don't have utility mode, singlenode mode should not support
	 * utility mode either.
	 */
	if (Gp_role == GP_ROLE_SINGLENODE && string_to_role(newval) == GP_ROLE_UTILITY)
	{
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
					errmsg("utility mode is not supported in ")));
	}

	Gp_role = string_to_role(newval);

	if (IS_UTILITY_OR_SINGLENODE(Gp_role) && MyProc != NULL)
		MyProc->mppIsWriter = false;
}

Singlenode role should not be used to connect to any of the cluster node either. Can be taken into consideration along with this issue.