Option to not change partitions of existing topics

[navdeepsekhon]

**Describe **
When kafkaer is run against a cluster where managed topic already exists, kafkaer will update the topic configs based on current config.json. In case of change in partition count, it will increase the partition if new number is higher or throw an exception if new number is lower. The user may not always want to change the partitions of existing topics.

Implement a new option --preserve-partition-count which when used, kafkaer will not change the current partitions and instead just log a warning.