tasks: Use '_' instead of '-' in names

Python modules are not allowed to contain hyphens
in the name.  Using hyphens only works here because
we're using low level __import__.

Change run_tasks to replace '-' with '_' when reading
configs, and rename the task modules to have valid
python module names.

Aside from general hygiene, the motivation to do this
is to allow one task to build on code in another
task by importing it.

Signed-off-by: John Spray <john.spray@inktank.com>
(cherry picked from commit 79dfe2cbefbbf14b69ae10c44a89a4fe45fedbe5)