Change use.hcm.sites from system property to cms.war properties file entry

The current requirement to set "legacy mode" with a system property may be too burdensome for existing customers upgrading to 13.0, especially if system properties set for an environment are controlled by different people than the developers building a distribution. An alternative approach would be to load this property from a properties file embedded in the war file that initially loads the repository. This would allow the same developers to both adjust the property and the location of HCM config files affected by the property.