Relates to https://bonitasoft.atlassian.net/browse/BR-28

* Change Jenkinsfile to use the new sqlserver docker image
* Fix server xa conf + client driver version
* dropping the database fails sometimes. For that, do not run all testMigration_x_y_z steps, but only the last known steps

It was not mandatory in MySQL 5.5. In this version, we can run an alter table
(drop default column value) operation even if no default is supported or
no default is set on the column.

Checking the existence of default value is required for Mysql 5.6+
On these versions, for column types that not support default value (BLOB,
TEXT, GEOMETRY or JSON), an error occurs when trying to alter the table
for default column value removal.

* Readme: fix database arguments to pass to run tests

Closes [BS-18534](https://bonitasoft.atlassian.net/browse/BS-18534)

Available for oracle, mysql and postgres for both integration and migration tests.

Documentation: explain how to choose the the db used by tests
We now have 2 cases:
  - when using docker container started by the build
  - sqlserver which still requires an external db to run
Also document how to run a single test from within an IDE.

In addition, when depending on an external database server for tests,
drop the created user and database to let the environment clean as before
the tests.
To do so, the drop part of the cleanDb task has been reworked to fix some
issues (duplication, missing or extra parts).

As part of this change, also fix an issue on postgres that prevented to
use a non default database (not running on localhost with default port
and with db name equal to bonita)
The postgres specific DB parameters (required by XA drivers) were missing when launching
Bonita Engine.

Various improvements in Jenkinsfile:
Fix: remove extra scm checkout when running db tests

Sqlserver: use the 2016 instance

Improvements:
  - use specific user and database name for master, dev and pr to avoid
  clash if builds are run in parallel
  - do not run db tests inside the node which run the intial build. This
  make consume an extra node for nothing (the initial node was waiting for
  nodes used for parrallel tasks before completing where it can complete
  right after the build finishes)

* jenkinsfile: do not compute dbConnectionSettingValue all the time

* readme: postgres running test example

* readme: integration tests with sqlserver docker run manually

Covers [CI-286](https://bonitasoft.atlassian.net/browse/CI-286)

Clarify alternate db usage examples
Provide infos about debug flags
Explain how to only build the subscription project
Remove the empty FAQ doc file

The gradle scripts files have no extensions.

refactor project structure to avoid code duplication when adding a step.

Instead of having a project for each test step, we have only one project
with multiple source sets, all tests are compiled with last version of
the engine but are run using the version of the step.
there is a filler sourceset and an enginetest source set containing code
to prepare the migration test and one to validate the engine was
migrated.
migration test steps are generated for each engine version.

the task integrationTest run all migration test and gradle build launch
only normal tests.

in order to be able to build community in standalone mode the plugin is in
community/buildSrc and linked in the root folder. See info about symlinks on Windows.

use axion release plugin:
It allows to tag alpha/beta/rc with gradle commands. see README.

use `gradle publish` to push artifacts to repositories.rd.lan

update readme with how to run the migration

Added section "How to tag" and "Add a migration step"

typo fix