[DISCUSS] Migrate from 1.2 to 2.0

Tracked as SYNCOPE-753.

Essentially, we need:

1. a tool generating (mostly) valid *Content.xml in 2.0 format, taking a 1.2 content.xml (ideally exported from a 1.2 deployment)
2. a "Syncope" ConnId connector which can be used to pull users and groups from an existing 1.2 deployment
3. a reference migration procedure to follow

The idea is that, given an existing 1.2 deployment, one will:

1. generate a new 2.0 project
2. export content.xml from 1.2 deployment
3. run the migration tool to convert most of content.xml into some MasterContent.xml - there are some items which cannot be directly migrated, as virtual schemas and related mappings
4. put the generated MasterContent.xml under core/src/test/resources/domains in the new 2.0 project
5. run the new 2.0 project in embedded mode while watching all logs - if errors are found, make appropriate corrections into core/src/test/resources/domains/MasterContent.xml - this might involve migrate 1.2 customized classes into their respective 2.0 counterparts
6. once errors are gone, access the admin console in embedded mode and look if every configuration item is at its place (schema, connectors, resources, mappings, notifications, ...)
7. manually add the items that cannot be automatically migrated
8. if using delegated administration, reconstruct roles and entitlements under the  new security model
9. verify that all operations (CRUD / propagate / synchronize / push) about users and roles in 1.2 are now working (CRUD / propagate / pull / push) with users and groups in 2.0
10. export the working configuration and save it as core/src/test/resources/domains/MasterContent.xml

At this point:

1. add the "Syncope" ConnId connector to the 2.0 project
2. start in embedded mode, define connector instance and resource for such a connector - you will need to connect to the internal storage of the 1.2 deployment
3. pull users and / or groups from the newly created resource