Submitting answer to my own question due to the fact that nobody seems to have an answer for this and I am sure someone else will eventually find it useful as well, because all the other examples of how to do this were for the old maven-jetty-plugin (<7.x).
After digging through Jetty's code for a while, I found out that the variable had been renamed as shown below:
<build>
<plugins>
<plugin>
<groupId>org.mortbay.jetty</groupId>
<artifactId>jetty-maven-plugin</artifactId>
<version>${version.jetty}</version>
<configuration>
<webAppConfig>
<contextPath>/foo</contextPath>
<sessionHandler implementation="org.eclipse.jetty.server.session.SessionHandler">
<sessionManager implementation="org.eclipse.jetty.server.session.HashSessionManager">
<!-- Disable url sessions using JSessionID -->
<sessionIdPathParameterName>none</sessionIdPathParameterName>
</sessionManager>
</sessionHandler>
</webAppConfig>
</configuration>
</plugin>
<plugins>
</build>