Merge pull request #1580 from jboeuf/tsi_properties_cleanup

src/core/security/security_connector.c

src/core/tsi/ssl_transport_security.c

src/core/tsi/ssl_transport_security.h

src/core/tsi/transport_security.c

src/core/tsi/transport_security.h

src/core/tsi/transport_security_interface.h

test/core/tsi/transport_security_test.c