Use pytest instead of nosetests

Nose has been in maintenance mode for the past several years
and pytest is a good replacement.

Other changes:
- Replace deprecated assertRegexpMatches with assertRegex
- Replace deprecated assertRaisesRegexp with assertRaisesRegex
- Replace deprecated SafeConfigParser with ConfigParser
- Force reinstall pytest and mock in tox virtualenv. This is because
  the globally installed packages may not work as expected(occured in
  jenkins job).

JIRA: RHELCMP-1619
Signed-off-by: Haibo Lin <hlin@redhat.com>

setup.py

@@ -66,5 +66,5 @@ setup(
         "dogpile.cache",
     ],
     extras_require={':python_version=="2.7"': ["enum34", "lockfile"]},
-    tests_require=["mock", "nose", "nose-cov"],
+    tests_require=["mock", "pytest", "pytest-cov"],
 )