Demisto-sdk

* Added a BC check for incident fields - changing from version is not allowed.
* Fixed an issue in create-content-artifacts where scripts in Packs in TestPlaybooks dir were copied with a wrong prefix.

* Added a validation that incident field can not be required.
* Added validation for fetch incident parameters.
* Added validation for feed integration parameters.
* Added to the *format* command the deletion of the *sourceplaybookid* field.
* Fixed an issue where *fieldMapping* in playbook did not pass the scheme validation.
* Fixed an issue where *create-content-artifacts* did not copy TestPlaybooks in Packs without prefix of *playbook-*.
* Added a validation the a playbook can not have a rolename set.
* Added to the image validator the new DBot default image.
* Added the fields: elasticcommonfields, quiet, quietmode to the Playbook schema.
* Fixed an issue where *validate* failed on integration commands without outputs.
* Added a new hook for naming of v2 integrations and scripts.

* Fixed an issue where *create-content-artifact* was not loading the data in the yml correctly.
* Fixed an issue where *unify* broke long lines in script section causing syntax errors

* Added *generate-docs* command to generate documentation file for integration, playbook or script.
* Fixed an issue where *unify* created a malformed integration yml.
* Fixed an issue where demisto-sdk **init** creates unit-test file with invalid import.

* Fixed an issue where demisto-sdk **validate** failed on modified scripts without error message.

* Fixed an issue with docker tag validation for integrations.
* Restructured repo source code.