Add the .mvn directory and set default Xmx value

Using the new feature introduced in Maven 3.3.1.
See
https://maven.apache.org/docs/3.3.1/release-notes.html#JVM_and_Command_Line_Options
for more details.

Possibly a long term better solution/first encounter for newcomers. When
people can't build Jenkins because of wrong default memory settings.
Like https://goo.gl/Co9kye

The "Xmx800m" was chosen after looking at last successful ci.j.o builds.
Max memory displayed by Maven being ~730MB. So putting 800 to leave the
build some room so that the GC has less chance to have to chime in too
often.