Introduce new runner option --wrapper.

Introduce a new command line option (--wrapper) to wrap
every new process created with help of module `avocado.utils.process`
inside a support script.

Wrappers can be global, so it will wrap every process or
you can set a wrapper to a specific program.

* Set wrapper to ltrace.sh on each process:
    avocado run --wrapper ltrace.sh sometest
* Set wrapper to ltrace.sh and binary myprog:
    avocado run --wrapper ltrace.sh:myprog sometest

You can provide multiples --wrapper options,
but only one global wrapper can be active at time.

For API, the class WrapProcess accepts an optional argument `wrapper`.
If it is defined, then use this parameter as the wrapper script.
If undefined, then it will look for the value inside
`runtime.CURRENT_WRAPPER` and use it.

If `runtime.CURRENT_WRAPPER` is None, then just behave like the
`SubProcess` class.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
examples.wrappers: Scripts to use with option --wrapper.

Scripts to use with command line option --wrapper.

* dummy.sh    Do nothing (use as a template).
* strace.sh   Run process on strace, output: strace.log.<pid>
* ltrace.sh   Run process on  ltrace, output: ltrace.log.<pid>
* perf.sh     Record process on perf, output: perf.data.<pid>
* valgrind.sh Run process on valgrind's memcheck, output: valgrind.log.<pid>

Usage:

  avocado run datadir --wrapper examples/wrappers/strace.sh
  avocado run datadir \
              --wrapper examples/wrappers/ltrace.sh:make \
              --wrapper examples/wrappers/perf.sh:datadir \

Signed-off-by: Rudá Moura <rmoura@redhat.com>

Valgrind.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
selftests: Add functional tests for --wrapper feature.

Add some functional tests to explore the --wrapper feature.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
docs+man: Add documentation for the --wrapper feature.

Add documentation manual page part for the command line
feature `--wrapper`.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #265 from ruda/utils_process_wrapper_V3

Introduce wrap process inside the tests [V3]