Modify get_stack_name function in heat_utils.py

Currently VNF can be upgraded from v1 to v2 with "tacker-db-manage" command[1].
However, LCM operation by v2 api for these upgraded VNF does not work.

In this operation, the stack related to target VNF is found by the get_stack_name function in heat_utils.py[2].
This function returns stack name like "vnf-" because stack name used in v2 api is like so.
But stack name used in v1 api is like "vnflcm_" or "<vnf_name>_<vnf_id>". It is different to v2 api.
Therefore the get_stack_name function can not find names of stack creted with v1 api.

Stack name cannot be renamed after it has been launch due to the heat specification[3].
Therefore, this blueprint aims to modify the get_stack_name function so that it can find stack name used in v1 api and LCM operation work for upgraded VNF with "tacker-db-manage" command.

[1]https://opendev.org/openstack/tacker/src/branch/master/doc/source/user/db_migration_tool_usage_guide.rst
[2]https://opendev.org/openstack/tacker/src/branch/master/tacker/sol_refactored/infra_drivers/openstack/heat_utils.py#L224
[3]https://docs.openstack.org/heat/latest/getting_started/create_a_stack.html#launching-a-stack