Add render_instance_selector helper to render instance selector

It works like the instance selectors in templates, except that it also
checks if multiple proposals are allowed for a barclamp, and doesn't
show anything if this is not the case.

This will make it easy to change, temporarily or not, the
allow_multiple_proposals of barclamps. And this avoids repeating these
three template lines several times in each barclamp template.
Sort commands in help out.

Otherwise randomized due to Ruby hash key insertion order.
Merge pull request #525 from vuntz/render_instance_selector

Add render_instance_selector helper to render instance selector
Merge pull request #528 from saschpe/release/pebbles/master-help-sorted

Sort commands in help out.