[service_discovery][jmx] fix yaml parsing

[sjenriquez]

What does this PR do?

YAML parsing for JMX-service discovery was broken after merging #2901

This small change fixes this bug.

[hkaj]

That should work yeah, but tests are not happy somehow:

======================================================================
ERROR: Test JMX configs are read and converted to YAML
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python2.7.9/lib/python2.7/site-packages/mock.py", line 1201, in patched
    return func(*args, **keywargs)
  File "/home/travis/build/DataDog/dd-agent/tests/core/test_service_discovery.py", line 593, in test_read_jmx_config_from_store
    jmx_configs = generate_jmx_configs(self.auto_conf_agentConfig, "jmxhost")
  File "/home/travis/build/DataDog/dd-agent/config.py", line 1144, in generate_jmx_configs
    _, (sd_init_config, sd_instances) = service_disco_check_config
ValueError: need more than 1 value to unpack
-------------------- >> begin captured logging << --------------------
config: INFO: Fetching service discovery check configurations.
config: DEBUG: Generating JMX config for: solr
config: ERROR: Unable to generate YAML config for solr
Traceback (most recent call last):
  File "/home/travis/build/DataDog/dd-agent/config.py", line 1150, in generate_jmx_configs
    yaml = config_to_yaml(check_config)
  File "/home/travis/build/DataDog/dd-agent/util.py", line 141, in config_to_yaml
    raise Exception('You need to have at least one instance defined in your config.')
Exception: You need to have at least one instance defined in your config.
config: DEBUG: Generating JMX config for: tomcat
--------------------- >> end captured logging << ---------------------```

[sjenriquez]

@hkaj Tests are passing now after updating our template. Not entirely sure how they were passing after #2901 was merged though.

[hkaj]

👌 squash the commits and it's good to go

[truthbk]