Add convenience environment to re-record tests

tox.ini's command specification doesn't expand globs (*) so one cannot
simply do:

    rm {posargs:cratonclient/tests/cassettes/*.yaml}

Instead one must be tricky and use find as we do in this commit. This
also allows folks to simply re-record only the cassettes they wish to
rerecord by doing

    tox -e rerecord -- TestHosts-test_create.yaml

As it will then execute

    find cratonclient/tests/cassettes -name 'TestHosts-test_create' \
        -delete

Or even use globs

    tox -e rerecord -- 'TestHosts*'

As that will translate to

    find cratonclient/tests/cassettes -name 'TestHosts*' -delete

This also ensures that new tests aren't recorded in our py27/py35
environments by setting the appropriate environment variable. Only the
rerecord environment may now record new cassettes.

Change-Id: I64e322548afc1b2d508fa129c0b151a8a91137a0

tox.ini

@@ -10,6 +10,7 @@ setenv =
    VIRTUAL_ENV={envdir}
    BRANCH_NAME=master
    CLIENT_NAME=python-cratonclient
+   BETAMAX_RECORD_MODE=none
 passenv = BETAMAX_* CRATON_*
 deps =
    -r{toxinidir}/requirements.txt
@@ -24,6 +25,15 @@ commands =
 [testenv:venv]
 commands = {posargs}
 
+[testenv:rerecord]
+setenv=
+    BETAMAX_RECORD_MODE=once
+whitelist_externals =
+    find
+commands =
+    find cratonclient/tests/cassettes/ -name '{posargs:*.yaml}' -delete
+    python setup.py test --slowest --testr-args='--concurrency=1'
+
 [testenv:cover]
 commands = python setup.py test --coverage --testr-args='{posargs}'