Fix findbugs violations in hwvtepsouthbound-impl

Fix findbugs violations in hwvtepsouthbound-impl

- Method call passes null for non-null parameter
- Nullcheck of value previously dereferenced
- Method ignores exceptional return value
- Inefficient use of keySet iterator instead of entrySet iterator
- Should be a static inner class
- Private method is never called
- Unread field
- Result of integer multiplication cast to long
- Dead store to local variable
- Method uses the same code for two switch clauses
- Write to static field from instance method
- Parameter must be non-null but is marked as nullable
- Redundant nullcheck of value known to be non-null
- Field not initialized in constructor but dereferenced without null check
- Useless object created

Change-Id: I78b60ea44fe72475b931bcb066a776fcd8dfa32d
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>