Update tox.ini files from release-tools gold copy

All OpenStack Charms now contain identical tox.ini files,
not to be modified or made unique within each charm repo.

This is to ensure consistency across charm repos in tox
target naming, approach and purpose, also giving the
charm dev and test experience additional consistency.

Also create empty dirs with .keep files where necessary.
Some classic charms have actions and/or lib dirs, and some
do not. In all classic charms, flake will now check those
dirs to ensure lint coverage of existing or future content.

Change-Id: I8fe6aac394448a090e58957ed0ccbd37f8481efc

lib/.keep

@@ -0,0 +1,3 @@
+  This file was created by release-tools to ensure that this empty
+  directory is preserved in vcs re: lint check definitions in global
+  tox.ini files. This file can be removed if/when this dir is actually in use.

tox.ini

@@ -8,6 +8,7 @@ skipsdist = True
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
+         CHARM_DIR={envdir}
          AMULET_SETUP_TIMEOUT=2700
 install_command =
   pip install --allow-unverified python-apt {opts} {packages}
@@ -24,7 +25,7 @@ deps = -r{toxinidir}/requirements.txt
 basepython = python2.7
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = flake8 {posargs} hooks unit_tests tests actions
+commands = flake8 {posargs} hooks unit_tests tests actions lib
            charm-proof
 
 [testenv:venv]