Pin bashate and allow for substitution

I want to release a new bashate, but I also don't want to risk
consuming it before fully testing it.  By pinning here, we can run all
our usual CI on new versions before accepting them.

Additionally, allow substitution of the bashate dependency via an
environment variable.  We can use this in a bashate test to substitute
a path to a checkout with any changes applied.

Change-Id: I165c4d66db8b7bdcff235ef7d8c99029637bb76a

tox.ini

@@ -8,7 +8,8 @@ usedevelop = False
 install_command = pip install {opts} {packages}
 
 [testenv:bashate]
-deps = bashate
+deps =
+   {env:BASHATE_INSTALL_PATH:bashate==0.3.1}
 whitelist_externals = bash
 commands = bash -c "find {toxinidir}             \
          -not \( -type d -name .?\* -prune \)    \ # prune all 'dot' dirs