Switch from nose to pytest (#5282)

* Use pipstrap to install a good version of pip

* Use pytest in cb-auto tests

* Remove nose usage in auto_test.py

* remove nose dev dep

* use pytest in test_tests

* Use pytest in tox

* Update dev dependency pinnings

* remove nose multiprocess lines

* Use pytest for coverage

* Use older py and pytest for old python versions

* Add test for Error.__str__

* pin pytest in oldest test

* Fix tests for DNS-DO plugin on py26

* Work around bug for Python 3.3

* Clarify dockerfile comments

setup.py

@@ -67,7 +67,9 @@ dev_extras = [
     'astroid==1.3.5',
     'coverage',
     'ipdb',
-    'nose',
+    'pytest',
+    'pytest-cov',
+    'pytest-xdist',
     'pylint==1.4.2',  # upstream #248
     'tox',
     'twine',