Support jUnit reports in paratest.server.

Yesterday, jUnit report support was added to start_test and piped through
nightly. paratest.server was missed and it can be used by nightly. The flags
and behavior in paratest.server now match start_test.
Merge pull request #1032 from thomasvandoren/add-junit-to-paratest

Support jUnit reports in paratest.server.

Yesterday, jUnit report support was added to start_test and piped through
nightly. paratest.server was missed and it can be used by nightly. The flags
and behavior in paratest.server now match start_test.