config: kvstore visitors refactoring

config: kvstore visitors refactoring

[Feature]       kvstore and kvjson visitors refactoring
[Cause]         N/A
[Solution]      1. All KVStore methods accepting non-string arguments has
                been moved to the FromKVStoreVisitors.
                2. FromKVStoreIgnoringVisitor has been added for loading
                incomplete config from KVStore.
                3. FromKVJsonVisitor has been removed. As replacement
                FromKVStoreIgnoringVisitor is now called right after
                FromJsonVisitor.
[Verification]  Build, install, run tests

Change-Id: I9d77388830c6e2914b1e1d91dcad211c6ae7059e