Fix config transaction handling in netconf.

Fix config transaction handling in netconf.

When config manager fails a transaction because of optimistic lock failure, netconf
must clean up this transaction as well. Fix TransactionProvider to abort transaction
if commit fails. Only exception is ValidationException where user should
have an option to fix the configuration and commit again.

Turn off netconf client reuse in persister: this narrows the possibility of
hitting concurrent modification with other config transactions. Each retry
to push a snapshot is now done with a newly obtained client, so new transaction
will be started each time.

Change-Id: I4d49fea10a682e4d7cd85ab49f2b78be63a84c02
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>