Kolla-ansible is currently installing mariadb
cluster on hosts defined in group['mariadb']
and render haproxy configuration for this hosts.

This is not enough if user want to have several
service databases in several mariadb clusters (shards).

Spread service databases to multiple clusters (shards)
is usefull especially for databases with high load
(neutron,nova).

How it works ?

It works exactly same as now, but group reference 'mariadb'
is now used as group where all mariadb clusters (shards)
are located, and mariadb clusters are installed to
dynamic groups created by group_by and host variable
'mariadb_shard_id'.

It also adding special user 'shard_X' which will be used
for creating users and databases, but only if haproxy
is not used as load-balance solution.

This patch will not affect user which has all databases
on same db cluster on hosts in group 'mariadb', host
variable 'mariadb_shard_id' is set to 0 if not defined.

Mariadb's task in loadbalancer.yml (haproxy) is configuring
mariadb default shard hosts as haproxy backends. If mariadb
role is used to install several clusters (shards), only
default one is loadbalanced via haproxy.

Mariadb's backup is working only for default shard (cluster)
when using haproxy as mariadb loadbalancer, if proxysql
is used, all shards are backuped.

After this patch will be merged, there will be way for proxysql
patches which will implement L7 SQL balancing based on
users and schemas.

Example of inventory:

[mariadb]
server1
server2
server3 mariadb_shard_id=1
server4 mariadb_shard_id=1
server5 mariadb_shard_id=2
server6 mariadb_shard_id=3

Extra:
wait_for_loadbalancer is removed instead of modified as its role
is served by check already. The relevant refactor is applied as
well.

Change-Id: I933067f22ecabc03247ea42baf04f19100dffd08
Co-Authored-By: Radosław Piliszek <radoslaw.piliszek@gmail.com>