Implement a deploy scripts search path and allow absolute paths to the script. Fixes #347.

* Implemented a deploy scripts search path for which the order is:
  * <config-file.path>/cloud.deploy.d/
  * <salt-cloud.package>/deploy/

* The user can also specify additional search path entries by providing `deploy_scripts_search_path` in the configuration file. It can be a string(a single path) or a list(multiple search paths). Non existing paths are not searched for scripts.

* If the user provided a full path to the deploy script, it's used and no additional searches are made.
Use `salt.utils.fopen()` instead of plain `open()`.
Minor formatting + PEP-8 changes.