Fix configuration for tox 2.0.x
Fix configuration for tox 2.0.x

Using latest tox 2.0.x our tox jobs fail with following error:

tox.ConfigError: ConfigError: substitution key 'envbindir' not found

So, replace substitution key 'envbindir' with 'envdir' with suffix '/bin'.

Change-Id: I51a4aa7d66cf24ec89cd2c87c9ce902b919ba4b1
Closes-Bug: #1455066

sniperganso/python-manilaclient,sniperganso/python-manilaclient