Logging improvements

Improve logging output and make consistent use of log levels in
plugin code.

Log messages for the archive and plugin classes now use their
own class-level logging interface to prefix messages with a
label indicating the message source, e.g.:

  [plugin:yum] collecting output of 'yum -C repolist'
  [archive:TarFileArchive] finalizing archive '/var/tmp/sosreport-f20-vm1-20140406162540'

Convert existing plugin and archive code to use the new
interfaces.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>