I've just solved this exact problem on my Ubuntu 14.04 with Eclipse Mars 2.
This could happen when Eclipse is not finding Tomcat's configuration files where they are expected to be. This place is in
$eclipse_workspace_folder/$version_of_your_tomcat_server_at_localhost/
(by default if you didn't changed server's name). So you have to copy all the files under your $tomcat_installation_folder/conf/* to the workspace server's folder.
But it was easier to just remove the server from your server list and add it again. Eclipse will automatically recreate all these files again into the proper folders. Like in the picture below:
In my case I've downloaded tomcat-8.0.35 from the website, so the configuration files needed are in /opt/apache-tomcat-8.0.35/conf/ filesystem.
Just delete the desired server from the Servers view (Window -> Show View -> Servers) and then go to Window -> Preferences -> Server -> Runtime Environment -> Add and add the server again.