[pytest]: add tacacs ro user test

[lguohan]

• put tacacs configuration int tacacs fixture
• fix loganalyzer disable logic

Description of PR

Summary:
Fixes # (issue)

Type of change

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

Approach

How did you do it?

How did you verify/test it?

johnar@e1ea830bda60:/data/sonic-mgmt/tests$ py.test --inventory veos.vtb --host-pattern all --user admin -vvv --show-capture stdout --testbed vms-kvm-t0 --testbed_file vtestbed.csv --disable_loganalyzer --log-level=debug --log-file=aaa tacacs/test_*.py | tee abc.txt
============================= test session starts ==============================
platform linux2 -- Python 2.7.12, pytest-4.6.6, py-1.8.0, pluggy-0.13.1 -- /usr/bin/python
cachedir: .pytest_cache
ansible: 2.8.7
rootdir: /data/sonic-mgmt/tests, inifile: pytest.ini
plugins: ansible-2.2.2
collecting ... collected 2 items

tacacs/test_ro_user.py::test_ro_user PASSED                              [ 50%]
tacacs/test_rw_user.py::test_rw_user PASSED                              [100%]

=============================== warnings summary ===============================
tacacs/test_ro_user.py::test_ro_user
  /usr/local/lib/python2.7/dist-packages/pytest_ansible/module_dispatcher/v28.py:70: UserWarning: provided hosts list is empty, only localhost is available
    warnings.warn("provided hosts list is empty, only localhost is available")

-- Docs: https://docs.pytest.org/en/latest/warnings.html
==================== 2 passed, 1 warnings in 33.81 seconds =====================

Any platform specific information?

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

Documentation