Render some username and password in creds fixture

[wangxin]

Description of PR

Summary:
Fixes # (issue)

Some username and password retrieved using the creds fixture
may have reference to other variables. If the username and
password are not consumed by ansible, they would not be
rendered. This fix renders some of the username and password
in the creds fixture.

Type of change

• Bug fix
• Testbed and Framework(new/improvement)
• Test case(new/improvement)

Approach

What is the motivation for this PR?

The ptf testing in advanced reboot failed because the dut_username and dut_password contain un-rendered variables.
This PR is to fix the issue.
The test_wr_arp.py also need to be fixed.

How did you do it?

Render the sonicadmin_user and sonicadmin_password variables using Jinja2 template and gathered vars of the duthost in the creds fixture.

How did you verify/test it?

Any platform specific information?

Supported testbed topology if it's a new test case?

Documentation

[wangxin]

I think yes, it can be a shared function or method. Currently creds is a fixture. The creds fixture just retrieves all the pre-defined credentials in scope from ansible group and host var files. Rendering the credentials need input of variables specific to some host. It could be ptfhost, duthost or others. So, just enhancing the creds fixture may not be a good option. We can make it shared if we need this somewhere else in the future. By then we can have better understanding of where is the better place to add the shared function.

[bingwang-ms]

How about add another fixture to retrive sonic_admin_user and sonic_admin_password? I think this feature is needed by a number of cases. And I guess others may make the same mistake when use creds to get username and password.

[neethajohn]

What about the test_wr_arp.py? Is it rendered there correctly?

[yxieca]

Let's move on with this change first. Then improve later.

[tahmed-dev]

can the template be done once in the creds fixture itself?

[yxieca]

I believe fixture can be a class. But let's move on to fix the issue then improve.

[wangxin]

What about the test_wr_arp.py? Is it rendered there correctly?

Thanks for bringing this up. Unfortunately, test_wr_arp.py needs update as well. It looks like there is a need to make it shared now. I think creds fixture is not the right place to add the Template rendering code. Such username/password are specific to different type of hosts. I'll add the code to the SonicHost class.
Please don't merge this PR for now. More updates are coming soon.

[wangxin]

Multiple places referred to creds['sonicadmin_user'] and creds['sonicadmin_password']. It turns out rendering these two variables in the creds fixture makes more sense. I have updated this PR in that way. Thanks for all the good suggestions!