self.avocado.plugins.runner: don't need to quote list of tests.

The parser for the run subcommands now accept one or more
tests ids (paths / aliases), don't need to quote the list
as a string.

Example:

* Before: avocado 'sleeptest failtest /usr/bin/true'
* After:  avocado sleeptest failtest /usr/bin/true

It's now possible to run avocado like `avocado run *.py`.

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