Add global statistic for stress tests

Adds shared memory between all stress action processes and the driver in order
to generate a global statistic. A new command line option is introduced
with this it is possbile to terminate an action if the
specifited number of execution is reached.

Implements: blueprint stress-tests

Conflicts:
tempest/stress/actions/create_destroy_server.py
tempest/stress/actions/volume_create_delete.py
tempest/stress/driver.py
tempest/stress/run_stress.py

Change-Id: I34765932ba93cd6d7f0df23ab97d9483eb459978
Merge "Add global statistic for stress tests"