Add command line option to print the actual test groups that are going to be executed

[ArturGajowy]

The use case is as follows:
In presto, we spin up a postgresql, msyql, and a cassandra container each time the tests are run in thesinglenode profile - even if no tests from their respective groups are run and the containers are effectively unused. To prevent starting of the containers we need to know if the respective tests are going to be run - and without support in tempto that would mean reimplementing a part of tepto/testng. Thus, we'd like the option to:

#assuming test groups foo,bar,baz:
$ ./tempto --print-actual-groups -g foo,bar
foo
bar
$ ./tempto --print-actual-groups -x bar
foo
baz

Then we can grep the groups and see which containers need to be run.

Notice that we'd like tempto to exit immediately after printing the groups.

CC @sanjay990