Cleanup of config needed in service_instance_test

[Praveen]

service_instance_test must remove configuration at end of test. Following crash seen due to some stale configuration.

praveen@praveen-VirtualBox:~/ctrlplane$ /home/praveen/ctrlplane/build/debug/vnsw/agent/test/service_instance_test
WARNING: Perftools heap leak checker is active -- Performance may suffer
[==========] Running 4 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 4 tests from ServiceInstanceIntegrationTest
[ RUN ] ServiceInstanceIntegrationTest.Config
log4cplus:ERROR No appenders could be found for logger (root).
log4cplus:ERROR Please initialize the log4cplus system properly.
service_instance_test: controller/src/vnsw/agent/oper/vn.cc:955: virtual DomainConfig::~DomainConfig(): Assertion `ipam_config_.size() == 0' failed.
Aborted (core dumped)