tempest improvements

Should be tested with https://github.com/crowbar/barclamp-git/pull/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.