Resolve ${solr.server.url} placeholder in prod file

* Because of the multiple PropertyPlaceholderConfigurer beans - root-context.xml
and solr-context.xml - the ${solr.server.url} placeholder was not being resolved by
the bean in root-context.xml and as such the app was throwing an exception on startup.
This has been fixed by setting "ignoreUnresolvablePlaceholders" to true - in the property
configurer in root-context.xml