Add Plugin method to create new command output directories

Some plug-ins want to store command output in subdirectories of
their 'sos_commands/<plugin>/' directory. Currently there are
various hand-rolled methods to build this in different plug-ins.

Add a single method to the base Plugin class that builds the
correct path string and recursively creates the directories via
os.makedirs() in a single step.

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