Standardize plugins settings, features and MIB formats

[edmocosta]

• Standardized the logstash-input-snmp and logstash-input-snmptrap settings, supported MIB formats and features.
  • Both plugins now support .dic (libsmi) and .yaml (ruby-snmp) MIB formats.
  • Added the oid_mapping_format option to configure how the OID field is mapped in the Logstash event.
  • The logstash-input-snmptrap now supports the logstash-input-snmp features: mib_paths, use_provided_mibs, oid_root_skip, oid_path_length.
• The logstash-input-snmptrap yamlmibdir was deprecated in favor of mib_paths.
• Updated the logstash-input-snmptrap docs to include the same logstash-input-snmp instructions on how importing MIB files.
• Refactored specs to remove all allow_any_instance_of usages (rspec/rspec-mocks/issues/1338)

[karenzone]

@edmocosta FYI, I'm adding myself as an assignee for tracking docs reviews--not taking credit for your awesome work. :-)