Add properties filename constants to Properties Manager

[niio972]

We need to had constant variables as done in UriGenerator :
e.g. PropertiesFileManager.getConfigFileProperty(PROPERTIES_SERVICE_FILE_NAME, PROPERTIES_SERVICE_BASE_URI);
instead of use raw string like :
PropertiesFileManager.getConfigFileProperty("service", "baseURI")

These constant variables must be in PropertiesFileManager.class .