log: refactor code, update arguments of Backend.log()

Change-Id: Ie9f9a603447fd779c7679386b358d28d54c4281a
log: update test code, use getDefaultLogLevel() when testing environment variables

Change-Id: I66d288aae2a75aad99dcdc5f416efaedfcd00bae
log: support syslog mode for console backend

Change-Id: I78019e2871649254e43227eca710cbf98f713d41