Substract prefix length from pty width in run() output
Avoid hanging output lines which might corrupt synchronized output
Prefix host specific error messages with [host] prefix
Fab host definition: New parameter for giving a hosts file

The fab commandline option "--hosts-file=<hosts file> can be given
to provide a simple text file containing hosts line by line.

These hosts will overwrite any previously set hosts (given on cmdline
or in the fabfile via the env hash.
Parallel execution: Handle NetworkError exceptins in child process

Python Exceptions thrown in the child process are obviously handled
by the multiprocessing lib that is showing a traceback. The handler
of the NetworkError exception in tasks:execute was not receiving the
exception

Since we don't want to see the trace we are handling the exception
in the child process, showing an error.

For the reason of backwards compatibility env.use_exception_for['network']
is evaluated in the same manner as in tasks:execute.
Fabric tasks: Adding a summary of hosts succeeded and failed

Per task there will be summary indicating on which hosts the task
has succeeded or failed.

Meant for getting parsed by a tool connected by a pipe.