Tox.ini is incompatible with tox 1.7.0

[Pavlo Shchelokovskyy]

Tox 1.7.0 was released recently. As heat does not has tox in it's test-requirements.txt, I just pip-installed the latest one, and now can not run `tox` command to run any tests, as the following error is produced:
Traceback (most recent call last):
  File "/home/pshchelo/.virtualenvs/heat/bin/tox", line 9, in <module>
    load_entry_point('tox==1.7.0', 'console_scripts', 'tox')()
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_cmdline.py", line 25, in main
    config = parseconfig(args, 'tox')
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 47, in parseconfig
    parseini(config, inipath)
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 281, in __init__
    config)
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 346, in _makeenvconfig
    vc.commands = reader.getargvlist(section, "commands")
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 518, in getargvlist
    commandlist.append(self._processcommand(current_command))
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 550, in _processcommand
    new_word = self._replace(word)
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 666, in _replace
    return RE_ITEM_REF.sub(self._replace_match, x)
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 662, in _replace_match
    return handler(match)
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 633, in _replace_substitution
    val = self._substitute_from_other_section(sub_key)
  File "/home/pshchelo/.virtualenvs/heat/local/lib/python2.7/site-packages/tox/_config.py", line 627, in _substitute_from_other_section
    "substitution key %r not found" % key)
tox.ConfigError: ConfigError: substitution key 'posargs' not found

When I downgraded tox to 1.6.1, the error was gone, and I could run the tests again.

As we have tox.ini in the repo, I think we should add tox to test-requirements.txt and either pin the version to 1.6.1 or pin it to >=1.7.0 and fix the above error.

[Wei Wang (damon-devops)]

many projects now are incompatible with tox 1.7