Since Rally team wants to support all Python 2.6, Python 2.7 and Python 3
envs and based on way of launching tempest(Rally install Tempest in virtual
environment), we can try to find and select a suitable python interpreter
for Tempest(Tempest designed only for Python 2.7). It can allow to support
`rally verify` stuff in all python environments.
Change-Id: If8c94c28119a34e60c00f7ef2cb9b94bc2e62004