Fix pyvenv detection when executing Rally from source

[dliappis]

Currently ./rally is unable to detect virtualenvs created with pyvenv (which is the case when pyenv is installed).

The reason is that while sys.real_prefix is defined with regular Virtualenv's (such as the one created with python3 -m venv venv or virtualenv -p python3 venv) it is not defined when pyvenv is in use. In other words, if a user has installed pyvenv locally or globally, which is a requirement for integration tests anyway, in some platforms this takes over when creating virtualenv's even with python3 -m venv venv.

No sys.real_path in Python3 when pyvenv is in use

$ pyenv shell 3.6.4
$ which python3
~/.pyenv/shims/python3
$ python3 --version
Python 3.6.4
$ python3 -m venv venv
$ source venv/bin/activate
(venv) dl@MS-7885 ~ $ python3
Python 3.6.4 (default, Mar 12 2018, 11:20:13) 
[GCC 7.3.1 20180130 (Red Hat 7.3.1-2)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.real_prefix
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'sys' has no attribute 'real_prefix'
>>> sys.base_prefix
'/home/dl/.pyenv/versions/3.6.4'
>>> sys.prefix
'/home/dl/venv'