Stretch support

- Support Debian Stretch (MariaDB debian/upstream + Galera)
- Doc updated
- New travis tests (with Vagrant)
- Drop support of mysql_innodb_additional_mem_pool_size
- Xtrabackup optionnal installation + package name updatable
- Fix many issues with replication (Ansible 2.3)
- Auto-retreive master IP address on tests
- MySQL tools per OS release

vars/Debian-jessie.yml

@@ -0,0 +1,7 @@
+mysql_tools:
+  - mytop
+  - percona-toolkit
+  - python-mysqldb
+  - mysqltuner
+
+mysql_default_xtrabackup_package: 'percona-xtrabackup'

vars/Debian-stretch.yml

@@ -0,0 +1,6 @@
+mysql_tools:
+  - percona-toolkit
+  - python-mysqldb
+  - mysqltuner
+
+mysql_default_xtrabackup_package: 'percona-xtrabackup-24'