Introduce ability to execute multiple bootstrap scripts at the same time during install workflow

[marangiop]

Tox config
basepython = python2.7
deps =
nose
networkx==1.9.1
cloudify-common==5.1.0
commands=nosetests --nocapture croupier_plugin croupier_plugin/tests/integration/workflow_tests.py:TestPlugin.test_bernoulli {posargs}

Croupier Version
Commit 461bcf68b09c35ecb4f0e6cf05c4701dd7a2d22b of branch permedcoe

Is your feature request related to a problem? Please describe.
I have a blueprint that specifies several, different bootstrap scripts to be executed on CESGA HPC front-end. The problem is that the bootstrap scripts are not executed in parallel, but sequentially one after each other. This behaviour can be easily reproduced with a local tox test trying to execute the blueprint_bernoulli.yaml and just changing the second job inside the blueprint such that the boostrap .sh script of that job is different from the boostrap .sh script of the first job (files with different names). You will see that the two bootstrap scripts of the two jobs are not created on the HPC at the same time. The first boostrap script will be created on HPC and executed, then the second, etc.

Describe the solution you'd like
The expected behaviour is that the different bootstrap scripts should all be created and executed simultaneously on the HPC.