Refactor shutdown-impl: add parameter to RPC, remove secret validation and masking.

Remove old secret as it does not work well with config-persister.
Add parameter maxWaitTime to shutdown RPC that specifies amount of time before
server is forcibly shut down.
Work around buggy yuma message sending xml element without inner text
node if blank value is specified.

Change-Id: Ief3fea74107349c5ff425ffdc184346981254bc4
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>