Rename requires files to standard names

From bug report:
Rename tools/pip-requires to requirements.txt and tools/test-requires
to test-requirements.txt. These are standard files and tools in the
general world are growing intelligence about them.

Change-Id: I5d3bad7654e4ce101d19b40215d8c1cc93dd9832
Fixes: Bug #1179008

tox.ini

@@ -6,8 +6,8 @@ setenv = VIRTUAL_ENV={envdir}
          LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C
-deps = -r{toxinidir}/tools/pip-requires
-       -r{toxinidir}/tools/test-requires
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
 [tox:jenkins]
@@ -19,7 +19,7 @@ commands = flake8
 
 [testenv:pylint]
 setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/tools/pip-requires
+deps = -r{toxinidir}/requirements.txt
        pylint==0.26.0
 commands = bash tools/lintstack.sh