Fix example_bigtable system test

[olegkachur-e]

Fix issues with bigtable system test:

• ensure correct id strings with proper length, as it might be issues depending on length of SYSTEM_TESTS_ENV_ID value.
• assign proper id values to resources used
• add constants for resources names limits

[VladaZakharova]

Please, for the task calls please add comments like this

        #TEST SETUP
        [create_instance_task, create_instance_task2]
        >> create_tables()
        #TEST BODY
        >> wait_for_table_replication_task
        >> update_clusters_and_instance()
        #TEST TEARDOWN
        >> delete_table_task
        >> delete_table_task2
        >> [delete_instance_task, delete_instance_task2]
    )

[moiseenkov]

Changes look good as well as the requested change is provided, and the system test is green on staging.

[olegkachur-e]

Closing as ready for the community