Depending of devstack_localrc.USE_PYTHON3 variable, rally package can
be installed at python2 env or python3 env.
Unfortunatelly, there is strange behaviour of zuul_v3+ansible+devstack
playbooks, so in case of trying to access USE_PYTHON3 variable, some
strange error happens.
To avoid using this variable, we can just call pip3 to check whether
rally is installed under python3 or python2 env.
Change-Id: I424a63c7b0effca47eecb0ec4dcfe5113b837be8