Improve dependency checking and installation
This makes several changes all toward the goal of fixing
curtin running on precise and better handing dependency installation.
* change launcher to stop calling 'install-deps' itself, but rather just
invoke the command it needs with the best python available
the curtin main now handles installing its own dependencies if
the user provides --install-deps.
* pack_install now calls 'curtin pack-install --install-deps'
* curtin/util and portions of curtin/commands/main.py are expected
to be runnable with any python with only standard library modules.
Lastly, in order to verify all this is functional we add a Precise test.
That required some work to the test suite to handle powering the
system off because cloud-init in 12.04 does not have power_state module.