consolidate basepath defaults into app.cfg

This way we will always get the proper default value, even if a config
file doesn't specify the paths explicitly (for example in tests).

Change-Id: Iaac02de38d680514a9978bd51917a13101753881
tidy up usages of /tmp in tests

Removed all hardcoded /tmp paths. There are a few things (like
basepath.harness) which we can't put in a dynamically created /tmp
directory, so they are moved to ./ when running tests from a git
checkout (with a corresponding .gitignore rule).

Also ensured that all temp files and directories are cleaned up. The
only gotcha is that the cleanup does not always happen if nose is
interrupted during a run.

The end result is that for a fully passing test run, nothing will be
left behind in /tmp.

Change-Id: Iff060ea5b45ca028af128fce98ac4e3b4b40782e
test_group_edit: fix timing issue with is_text_present

We've really got to stop using that function.

    FAIL: bkr.inttest.server.selenium.test_group_edit:TestGroupsWD.test_remove_self_admin_group
      /usr/bin/vim +394 src/bkr/inttest/server/selenium/test_group_edit.py  # test_remove_self_admin_group
        self.assertTrue(not is_text_present(b, 'admin'))
    AssertionError: False is not True

Change-Id: I8b370576c3cfca784080d61911c39eea0bb11fa4