Use fixtures in the tests

TempDir() makes sure all files created under it are deleted
at the end of the tests.
FakeLogger() hides logs when the test passes and displays logs
for failed tests.

part of bug 1177924
Change-Id: I07acb66daa1932d7864a5431f1b64570b747ce5a
Replace nose with testr

- add .testr.conf (in base dir and nova_tests)
- remove all references to nose
- adjust paths: A couple of tests would assume the current directory
  was ./tests/ but it is now ./
- don't run the tests in parallel as the db tests have one connection
  per test class.

nova_tests hackery:
It seems testtools/testr imports everything and I just don't see a way of
running things in tests/ and nova_tests/ seperately but in one .test.conf.
So if you want to use testr directly you will need to:
 testr run
 cd nova_tests
 testr run

part of bug 1177924
Change-Id: I41875dcf94463fa5f9c07a7840c37089226c59ad
Merge "Use fixtures in the tests"
Merge "Replace nose with testr"