Add variable interpolation capabilities to catalog.xml files

[antonio]

Like in web.xml and server.xml tomcat's configuration file it would be very helpful to have interpolation for variable passed to the JVM in catalog.xml files

For some details take a look at:
​https://tomcat.apache.org/tomcat-7.0-doc/config/

Tomcat configuration files are formatted as schemaless XML; elements and attributes are case-sensitive. Apache Ant-style variable substitution is supported; a system property with the name propname may be used in a configuration file using the syntax ${propname}. All system properties are available including those set using the -D syntax, those automatically made available by the JVM and those configured in the $CATALINA_BASE/conf/catalina.properties file.

The basic idea is define TDS services without to be tied to specific configurations

For example:

<service name="all" base="" serviceType="Compound" >
    <service name="odap" serviceType="OPeNDAP" base="/thredds/dodsC/" />
    <service name="http" serviceType="FileServer" base="/thredds/fileServer/" />
</service>

could be written like:

<service name="all" base="" serviceType="Compound" >
    <service name="odap" serviceType="OPeNDAP" base="${tds.context.path}/dodsC/" />
    <service name="http" serviceType="FileServer" base="${tds.context.path}/fileServer/" />
</service>