add bash completion script

This patch add a script rally.bash_completion which enables
auto-completion of rally CLI in bash environmnet when running:

python setup.py install
or
python setup.py develop

test:
rally [tab]
rally ta[tab]
rally task [tab]
rally task de[tab]

to-do:
adding a cache to bash-completion result could speed up auto-completion

Change-Id: Ia4c68ccf3faea73da87a93ce64b36545d9d8411c
Merge "add bash completion script"