avocado.utils.process: Execute commands on a subshell

For a number of reasons, the original implementation
called the processes with shell=False (subprocess default),
I noticed later that we lose things like the ability
to use pipes in commands executed and proper job
control and process handling.

So change that and use shell=True.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>