Add python3-dev to bindep and use py27for some envs

Installed my workstation with Ubuntu Xenial and then hit a few
snags running tox.

python3-dev is needed to build the python3 envs. Did not actually
test python3-devel (rpm) as I don't have such a system.

While python2.7 is the default, tox (on Xenial) itself uses
python3.5, hence 3.5 becomes the basepython and a number
of envs don't work.

Driveby, removed warning about /bin/echo in py3pep8

Change-Id: Ie880246dd757570b8e444da0e643f3c50093bdc5

bindep.txt

@@ -11,6 +11,8 @@ libffi-devel [platform:rpm]
 memcached
 python-dev [platform:dpkg]
 python-devel [platform:rpm]
+python3-dev [platform:dpkg]
+python3-devel [platform:rpm]
 rsync
 xfsprogs
 libssl-dev [platform:dpkg]

tox.ini

@@ -45,6 +45,7 @@ commands =
 [testenv:py3pep8]
 basepython = python3
 install_command = echo {packages}
+whitelist_externals = echo
 commands =
   # Gross hack. There's no other way to get it to /not/ install swift itself
   # (which triggers installing eventlet) but also get flake8 installed.
@@ -53,6 +54,7 @@ commands =
   flake8 --filename=swift* bin
 
 [testenv:func]
+basepython = python2.7
 commands = ./.functests {posargs}
 
 [testenv:func-fast-post]
@@ -69,6 +71,7 @@ setenv = SWIFT_TEST_IN_PROCESS=1
 commands = {posargs}
 
 [testenv:docs]
+basepython = python2.7
 commands = python setup.py build_sphinx
 
 [testenv:api-ref]
@@ -77,6 +80,7 @@ commands = python setup.py build_sphinx
 # we do not use -W here because we are doing some slightly tricky
 # things to build a single page document, and as such, we are ok
 # ignoring the duplicate stanzas warning.
+basepython = python2.7
 commands =
   rm -rf api-ref/build
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html