[ finishes #54080837 ] allow syslog_aggregator can be disabled.

because to define property foo.bar in spec file implicitly defines foo, test `if properties.foo`
is always true. For this reason, the following 5 jobs, which have syslog_aggregator.address defined
in spec files, had problem when job syslog_aggregator is not enabled.

collector
health_manager_next
login
saml_login
uaa