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>
task/ceph: separate ceph.conf into function

This is so that other tasks can modify ceph.conf
as needed.

Signed-off-by: John Spray <john.spray@inktank.com>
tasks/mds_journal_migration: create.

This is for testing the changes to CephFS
journal format, especially the migration
of journals from old to new format.

This is a teuthology task rather than being
a workunit script, because it involves
manipulating services and config during the
procedure.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #259 from ceph/wip-journal-tool

mds_journal_migration task