[tests/generic_config_updater] syslog test can be tested independently (

#4839)

Summary:
Make syslog test run independently; add rollback test.

What is the motivation for this PR?
To avoid testing on one test case that is dependent on the previous test.

How did you do it?
Setup prerequisite environment for each test and rollback to original setup

How did you verify/test it?
Run test of sonic-mgmt/tests/generic_config_updater/test_syslog.py on KVM