Remove bad xml reports option.

* `xmlrunner` proved to be a way better choice than `junitxml` and it's even parsable by Bamboo. The output of `junitxml` is not.
* The output directory can be tweaked with the environment variable `SALT_XML_TEST_REPORTS`.
Renamed environment variable and create directory if it does not exit.

New environment variable name is `SALT_XML_TEST_REPORTS_DIR`.
Merge pull request #3567 from s0undt3ch/develop

Drop recently added JUnit report support.