Despite this question being rather old, I had to deal with a similar warning and wanted to share what I found out.
First of all this is a warning and not an error. So there is no need to worry too much about it. Basically it means, that Tomcat does not know what to do with the source attribute from context.
This source attribute is set by Eclipse (or to be more specific the Eclipse Web Tools Platform) to the server.xml file of Tomcat to match the running application to a project in workspace.
Tomcat generates a warning for every unknown markup in the server.xml (i.e. the source attribute) and this is the source of the warning. You can safely ignore it.