[vxlamgr]: Change default return value to true in doTask() to avoid retry the unknown table request

[gord1306]

What I did
[vxlamgr]: Change default return value to true in doTask() to avoid retry the unknown table request

Why I did it
If the default value is false, the request will be retied next time. But those cases which cause return value be false are unknown table request, I thought this kind of requests should only write LOG instead of doing retry

How I verified it
Send incorrect table name request to doTask for vxlanmgr.cpp

[prsunny]

Looks to be trivial. Its ok either way. ConfigDB validator is in progress where the expectation is swss only gets correct config.

[gord1306]

@prsunny, I saw the Configuration validation be list in the community meeting minute. Have you opened any documents for the design spec now? In fact, I am trying to implement gtest unit test cases for vxlanorch and vxlanmgr, and if those validations are done before orchagent, I will consider whether to skip these kinds of test cases