Now that the server team has switched to using enterprise-rhel-62-64-bit-dynamic-required as its main required build variant for running tasks, we should update the default variant in burn_in_tests.py to reflect this.

As a server engineer,
I want burn_in_tests.py to default to using enterprise-rhel-62-64-bit-dynamic-required when I run it,
so that the configuration needed to generate tests is pulled from the correct variant.

AC:

• DEFAULT_VARIANT is set correctly in burn_in_tests.py: https://github.com/mongodb/mongo/blob/master/buildscripts/burn_in_tests.py#L56