Added debug statement when probed TCP port is already used.
Call before_boot and after_postinstall hooks if available.
Added build hooks:
* before_create, after_create, after_up, 
* wait_ssh_enabled, after_command_sequence, after_postinstall
Tiny formatting fixes.
Load a definition only once and cache it afterwards.
This is required to maintain state in the definition.
E.g. toggling a flag in the build hooks.
* Prevent environment reinitialization.
* Made (current) provider accessible from environment.
* Made box accessible from the definition.
Added build hooks documentation.
Renamed hook :wait_ssh_enabled to :before_ssh.
Merge pull request #686 from Drachenfels-GmbH/master

Builds hooks & easy access to provider and box