Add ability to install rbenv independently

- Can now install rbenv separately. Can be used as a requirement in a
  state for installing rbenv plugins using the git or file states
Fix description of method and whitespace lost
Add event function to state runner for light event interaction via CLI
salt.cloud.lxc: Optimized salt client
Safebelts for lxc provisionning:

Cloud:
    - Allow to use alternate config prefixes
    - Filter default options

LXC module:
    - Filter default options in create
Report errors when preparation fails

Previously salt daemons could fail to start without giving any
reason, when (for example) chown failed during the startup.
This change doesn't fix anything, but at least reports the error
back to the user.
salt.states.archive: multiple improvments

- add a new keep paramater to optionnaly not delete the archive after extraction
- Fix the traceback in case of tar error by correctly returning a state return mapping
- Correctly handle extra tar options if any by adding defauts ones if only needed
- Pep8
- When silently extracting with tar, be sure that the no output stuff won't be
  handled as a failed extraction.
Add pip_exists_action argument to virtualenv state

If you have some editable packages in a managed virtualenv than
you should be able to set the default action of pip for existing
paths.
lxc: saltify -> bootstrap; lxc & seed: doc improvements
salt.modules.seed: typo
[salt.cloud] adding volumes to cloud states
[salt.cloud] fix volume_list in SaltNova
[salt.cloud] nova cloud driver has the provider injected
Create one map for all volume actions
volume_list is now a dictionary

Instead of being a volume object, it returns everything so that we don't
have to generate a response dictionary anymore.
Clean up comments for volumes in cloudclient
Add cloud states for volumes
list_nodes needs to be in SaltNova

for deleting with salt-cloud
Use class for server information

Like libcloud
volume_action doesn't always need names

for example, volume_list