Prompt patterns are now bound to trigger.netdevices.Vendor objects

+ Vendor objects now have a prompt_pattern attribute
+ All prompt patterns are now defined in settings.py:

  - Vendor-specific: settings.PROMPT_PATTERNS
  - IOS-like: settings.IOSLIKE_PROMPT_PAT
  - Fallback: settings.DEFAULT_PROMPT_PAT

+ New settings still need to be documented.
+ All execute_ functions have been simplified to eliminate hard-coding
  of vendor checking wherever possible