lvm: Check for errors when running commands
lvm: Fix check for existence of logical volume
drbd: Enable the boot.lvm service in crowbar-pacemaker

Since we use LVM for DRBD, we need to make sure that boot.lvm is
enabled.
drbd: Move resource recipe to a LWRP

What we really want is a LWRP that we can call to create a drbd
resource. This is much more flexible.
Add LWRP in crowbar-pacemaker to ease the creation of drbd devices

What's tricky here is that we expect the resources to be defined (as
attributes to the nodes) after the recipe will have been compiled.

That means two things:

 - users of crowbar_pacemaker_drbd will need to run the create action in
   the compile phase

 - we need a hacky LWRP to actually do the actions in crowbar-pacemaker
   in the execute phase. This is the
   crowbar_pacemaker_drbd_create_internal LWRP.
drbd: Update examples for non-crowbar usage
Ensure that the kernel module we need for the drbd filesystem is loaded

We know that at least for xfs, this is needed, or on boot, we might end
up with:
 ERROR: Couldn't find filesystem xfs in /proc/filesystems
(it's then loaded later and works fine)
Merge pull request #58 from vuntz/drbd-helper

Add a crowbar_drbd definition to ease the creation of drbd devices