avocado: Print config file paths in datadir and list subcommands

It is useful information to the user where the config file
is, to reinforce where avocado is picking the default values.
So add it to the output of `datadir` and `list` subcommands.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugin: Visual plugin output tweaks

Get to a more toned down output for the subcommands
'datadir', 'plugins' and 'list'.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.plugin: Add parser attribute to Plugin object

This way we define more clearly the attributes of the
plugin object (many of them will implement a parser
object).

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins: Split sysinfo and test_lister plugins

Put the sysinfo and test_lister plugins into separate
plugin files. Although not strictly necessary, the
neat 1 plugin - 1 source file relation makes things
organized and easy to understand.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #291 from lmr/plugin-tweaks

Plugin tweaks