remove(or deprecate) 'timezone' item in 'inventory.ini'

[bb7133]

The 'timezone' item in the current configuration file can be used to determine systemTZ and system_time_zone at the first bootstrap stage(eg. start from zero) of a TiDB cluster. However, after the first bootstrap, those two variables become read-only and never change for a cluster, more detailed information can be referenced as follows:

• https://github.com/pingcap/tidb/blob/master/docs/design/2018-09-10-adding-tz-env.md
• server: set system_time_zone from system_tz tidb#13934

So, end-users may try to change 'timezone' for an existing TiDB cluster, for example, when doing a rolling-upgrade operation, but found nothing changes: 'timezone' can be confusing and misleading.

For the reason above it's better to remove 'timezone' in TiDB-Ansible, which lead the end-users to the configurations with explicit semantic, like the 'time_zone' system parameter in TiDB.

[tiancaiamao]

The system_time_zone is reasonable.
If the user doesn't specify @@global.time_zone and @@session.time_zone, which time zone should TiDB use?

[tiancaiamao]

Maybe we can change the ansible configure to something like:

bootstrap-timezone: utc+8  // do not edit after the cluster bootstrap, you can use SQL 'set @@global.time_zone' instead

[liubo0127]

Maybe only Remove timezone in inventory.ini, the timezone of the database depends on the local timezone when bootstrap.

[bb7133]

The system_time_zone is reasonable.
If the user doesn't specify @@global.time_zone and @@session.time_zone, which time zone should TiDB use?

Certainly, the system_time_zone will be kept. I think using 'bootstrap-timezone' is fine.