Migrate server_roles fixture from old to new code

This should get us a -1 on jenkins, because `test_pxe_provisioning.test_pxe_provision_from_template` is failing on server roles setup.

What is really happening is that the old code is not able to work around a different accordion being open in the  `Configure / Configuration` section - it tries to click the 'Server' tabbutton and fails -> we need `force_navigate` for that -> we have to use the new code.

Tested using downstream & upstream, test `test_mine_d` should fail (on purpose):
```
import pytest

@pytest.mark.fixtureconf(clear_roles=True)
def test_mine_a(server_roles):
    pass

@pytest.mark.fixtureconf(clear_roles=True)
def test_mine_a_again(server_roles):
    pass

@pytest.mark.fixtureconf(set_default_roles=True)
def test_mine_b(server_roles):
    pass

@pytest.mark.fixtureconf(server_roles='+rhn_mirror automate')
def test_mine_c(server_roles):
    pass

@pytest.mark.fixtureconf(server_roles='-nonexistent_role')
def test_mine_d(server_roles):
    pass

@pytest.mark.fixtureconf(server_roles_cfmedata=['server_roles', 'all'])
def test_mine_e(server_roles):
    pass
```

Also, `set_server_role` is removed and, where used, replaced with `set_server_roles` instead (in 1 file).