You can use relative paths. For example "../standalone/configuration/atx-tax-users.properties" if you are starting AS7 from the $JBOSS_HOME/bin directory.
Alternatively you can include the properties file in the WEB-INF/classes folder of your web application. It would then be included in the classpath so you can set the option to "atx-tax-users.properties".
Ideally, you should use the properties files for simple setup such as testing etc. Do not use it in production. Use DB/LDAP login modules.
Ideally, you should place the properties files in the classpath of the deploying application. For web, simplest would be WEB-INF/classes.