As a first trial step of contributing storage layer codes to sai/stub discussed here,
this pull request enables useful error messaging by comparing given API parameters
and the VLAN states already created by former API calls.
For example, this patch allows stub_create_vlan to check if the given VLAN id is already in use,
by adding the storge layer code to store states.
Example: if vlan_api->create_vlan(10) is called twice this patch yields a warning:
[SAI_VLAN.WARNING] stub_sai_vlan.c- stub_create_vlan: Warning: given vlan_id (10) already exsits.