Fix: Tempest improvements

https://github.com/crowbar/barclamp-tempest/pull/58

Should be tested with crowbar/barclamp-git#31
1) Tempest will be installed into venv. This will help to avoid conflicts with client binaries such as following case:
tempest is installed on same node with nova-controller
nova-controller is shiped with python-novaclient ==1.2.2 but tempest is depend on <=1.2.1

2) Tempest will create prerequisites for ec2 testsuite.