Fixed ovs provider

The vs_bridge type needed:

  - A regex value to validate external_ids

  - An extra test for autorequire

The vs_bridge provider needed:

  - Openvswitch corresponding link to go
    down before being remove: #destroy

  - parameter external_ids had a typo and
    couldn't be setup properly during create

Standardization

 - Replaced 'optional_commands' with 'commands'
   and used no path to be more generic as
   puppet will search for best path

 - Types using :namevar good practice

 - Inline documentation: for all parameters
   and properties. Replaced @doc with desc

 - Comestic: some doubled quotes => single

Change-Id: I522888b1d2d147e07667344f9dfc9bcbc0c96668