Add release notes command to tox

Added command "tox -e newnote slug-goes-here" to create a new release note.
Fixes error that occurs when reno is not installed globally.
Also changed contributor documentation to include this new command.

Change-Id: I4568163d63e389891e701a424720567d065f70b3

doc/source/contributor/adding_release_notes.rst

@@ -65,18 +65,12 @@ To create a release note for your change, use:
 
     $ reno new slug-goes-here
 
-If reno is not installed globally on your system, you can use it from venv
-of your manila's tox. Prior to running the above command, run:
+If reno is not installed globally on your system, you can use a tox
+environment in manila:
 
 .. code-block:: console
 
-    $ source .tox/py3/bin/activate
-
-Or directly as a one-liner, with:
-
-.. code-block:: console
-
-    $ tox -e venv -- reno new slug-goes-here
+    $ tox -e newnote slug-goes-here
 
 .. note::
 

tox.ini

@@ -31,6 +31,12 @@ commands =
          -b html releasenotes/source releasenotes/build/html
 allowlist_externals = rm
 
+[testenv:newnote]
+deps =
+-c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
+commands = reno new {posargs}
+
 [testenv:debug]
 commands = oslo_debug_helper {posargs}