[tests/generic_config_updater] Remove reset check for dhcp_relay test (…

…#5005)

Summary: 
Remove test reset check for GCU test cause GCU feature will take care of it

What is the motivation for this PR?
Dhcp_relay test: Remove reset check and rollback instead of reload. Use MACRO for each setup. Add try catch for each test to ensure checkpoint is removed after test.
Syslog test: Rename function. Setup code restructure. No logical change.

How did you do it?
Remove code related

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