Add helpers to be able to "synchronize" nodes in the cluster

When non-founder nodes call:
  PacemakerHelper.wait_for_mark_from_founder(node, cookbook_name, "pki-sync", node[:keystone]["crowbar-revision"])

they will be blocked until the founder calls:
  PacemakerHelper.set_mark_if_founder(node, cookbook_name, "pki-sync", node[:keystone]["crowbar-revision"])

This also comes with a LWRP, to enable synchronization in the second
phase of a chef run. It can be used to allow the founder to execute some
resources first. For instance:

  - before the critical code that the founder should run first:

    crowbar_pacemaker_sync_mark "wait-for-endpoints-mark" do
      mark "endpoints"
      revision node[:keystone]["crowbar-revision"]
      action :wait
    end

  - after the critical code that the founder should run first:

    crowbar_pacemaker_sync_mark "set-endpoints-mark" do
      mark "endpoints"
      revision node[:keystone]["crowbar-revision"]
      action :create
    end

If the revision parameter is not passed, then
node[cookbook_name]["crowbar-revision"] will be used, which is
convenient for crowbar integration.

The name of the crowbar_pacemaker_sync_mark can also be used to
automatically generate the mark parameter and make it optional. For
instance:

    crowbar_pacemaker_sync_mark "wait-test-def" do
      action :wait
    end

    crowbar_pacemaker_sync_mark "create-test-def" do
      action :create
    end
Add helper to synchronize all nodes

Previous work was working with a model of "one node goes first", but a
we also need a "wait for all nodes" model.

This adds a CrowbarPacemakerHelper.synchronize_on_mark helper, as well
as a :sync action for crowbar_pacemaker_sync_mark.
Add some documentation for synchronization helpers
Make default action for crowbar_pacemaker_sync_mark :sync

This is probably more useful than :wait.
Add a new :guess action for crowbar_pacemaker_sync_mark

This will guess the right action based on the name; this is something we
were doing to guess the mark, but we can go further and actually guess
the action too.

This is the default action.
Remove name of cookbook from synchronization marks

This is actually not useful because the name of the marks have to be
unique (otherwise, the various crowbar_pacemaker_sync_mark resources in
different cookbooks will conflict).
Add some doc for LWRP
Merge pull request #50 from vuntz/sync-marks

Add the synchronization helpers