utils: Add control-mechanism for trivial-httpd

Create a class in utils.py where ostree trivial-httpd
can be be started and stopped when the -toolbox process
is complete.  Also, if the -toolbox process terminates early,
the trivial-httpd is killed by PR_SET_PDEATHSIG

* utils.py
    - Created new class trivialHTTP that starts, stops
    - Added ctypes implemtation to help control early
      termination with PR_SET_PDEATHSIG

* installer.py and imagefactory.py
    - Removed previous implementation of trivial-httpd and
      replaced with calls to the util class.