- Introduce a RESOURCES_DIR similar to other *_DIR
- Add a resources_directory parameter to lookup_vendor() to avoid possible
  hardcoded directory and - if not provided - pick up the default
  RESOURCES_DIR.

(Otherwise ssh_keys are tried to picked up in in ${PWD} (and mostly
likely failing))

Commit id 9380c04 (probably
accidentally) removed EXPLOITS_DIR use leading to loading
routersploit/modules/exploits relative to the current directory
instead of the installed ones in MODULES_DIR.

Spotted by @lucyoa, thanks!

payloads should be listed/selected as `<architecture>/<payload>', not
`payloads.<architecture>.<payload>'.

Thanks to @lucyoa!