[vs]: Add option to specify platform name for DVS orchagent

[volodymyrsamotiy]

Signed-off-by: Volodymyr Samotiy volodymyrs@mellanox.com

- What I did
Added possibility to specify platform name for DVS orchagent.
It is needed in order to be able to enable platform specific "orchagent" code for testing purposes.

- How I did it
Added new fake_platform environment variable to specify platfom name for DVS orchagent.
If fake_platform is passed to the vs docker container then run orchangent with platform set to specified value.
If fake_platform is not defined then behavior is not changed and platform is set to vs.

- How to verify it

• Run vs container with the -e option and define fake_platform (e.g. -e fake_platform=mellanox).
• Go to vs container console and check that fake_platform is set: cat $fakeplatform.
• Check that orchagent was started with the platform set to specified value: cat /proc/`pidof orchagent`/environ.

- Description for the changelog
[vs]: Add option to specify platform name for DVS orchagent

[lguohan]

how is this coordinated with the vs test? we need to start the orchagent with mellanox platform while only run a subset of the vnet test. for all other tests, we still run under vs platform.

[volodymyrsamotiy]

Yes, it works in that way.
In VS container, orchagent started with mellanox platform only for bitmap vnet test.
For all other tests it works without any changes and platform is vs.

There are also changes to sonic-swss/tests/conftest.py, in this PR sonic-net/sonic-swss#792.
In "conftest.py" we have "DVS" fixture for pytest tests, which has scope set to "module".
As a result VS container is started per each pytest module (test suite).
These changes allow to start any test suite with the specified platform name by defining it in test suite file.
Defined platform is passed to DVS fixture and then it starts VS container with appropriate platform ENV variable.
So, eventually VS orchagent.sh reads that ENV variable and starts with required platform name.
(If platform is not defined by test suite then nothing to change and all works as before)

For example, in case of VNET test there are two test suits for both VNET implementations.
First one, test_vnet.py, is running with orchagent platform set to "vs" (as any other test).
Second one, test_bitmap_vnet.py, defines platform name, so it is running with orchagent platform set to mellanox.

$ pytest -v

    <--- <all tests are running with "vs" platform>

test_vnet.py::TestVnetOrch::test_vnet_orch_1 PASSED                                                                                                                                                                                  [ 16%]
test_vnet.py::TestVnetOrch::test_vnet_orch_2 PASSED                                                                                                                                                                                  [ 33%]
test_vnet.py::TestVnetOrch::test_vnet_orch_3 PASSED                                                                                                                                                                                  [ 50%]

    <--- <test in "test_vnet_bitmap.py" defines platform, so it is running with "mellanox" platform>

test_vnet_bitmap.py::TestVnetBitmapOrch::test_vnet_orch_1 <- test_vnet.py PASSED                                                                                                                                                     [ 66%]
test_vnet_bitmap.py::TestVnetBitmapOrch::test_vnet_orch_2 <- test_vnet.py PASSED                                                                                                                                                     [ 83%]
test_vnet_bitmap.py::TestVnetBitmapOrch::test_vnet_orch_3 <- test_vnet.py PASSED   

    <--- <all next tests are running with "vs" platform again>