XMP parsing: variable substitution should be the same as in JBoss

In JBoss' XML, we can say
value="${var1,var2,var3:8900}", which means that the value is that of system property var1 if set, else that of var2 if set, else that of var3 if set, or else 8900.

JGroups doesn't support a list of variables, it supports only one variable. Change this to make it parse the format supported by JBoss, too.