Helm: add support for secret rules and config

[cookandy]

Hi there!

This PR adds support for loading the Elastalert configuration and/or rules via Kubernetes secrets.

Three new variables were added to values.yaml:

• secretConfigName: name of the secret which holds the Elastalert config. Note: this will overwrite the generated config and must contain a full working config
• secretRulesName: name of the secret which holds the Elastalert rules. Note: this will overwrite the generated rules and must contain all rules in separate data fields
• secretRulesList: a list of rules to enable from the secret

It's worth mentioning that it was not possible to load a single secret with multiple rules separated by ---, it threw the error below. This is why secretRulesList was needed.

Index elastalert already exists. Skipping index creation.
Traceback (most recent call last):
  File "/usr/local/bin/elastalert", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.6/site-packages/elastalert/elastalert.py", line 2049, in main
    client = ElastAlerter(args)
  File "/usr/local/lib/python3.6/site-packages/elastalert/elastalert.py", line 140, in __init__
    self.rules = self.rules_loader.load(self.conf, self.args)
  File "/usr/local/lib/python3.6/site-packages/elastalert/loaders.py", line 115, in load
    rule = self.load_configuration(rule_file, conf, args)
  File "/usr/local/lib/python3.6/site-packages/elastalert/loaders.py", line 179, in load_configuration
    rule = self.load_yaml(filename)
  File "/usr/local/lib/python3.6/site-packages/elastalert/loaders.py", line 197, in load_yaml
    loaded = self.get_yaml(filename)
  File "/usr/local/lib/python3.6/site-packages/elastalert/loaders.py", line 526, in get_yaml
    return yaml_loader(filename)
  File "/usr/local/lib/python3.6/site-packages/staticconf/loader.py", line 168, in yaml_loader
    return yaml.load(fh, Loader=SafeLoader) or {}
  File "/usr/local/lib/python3.6/site-packages/yaml/__init__.py", line 114, in load
    return loader.get_single_data()
  File "/usr/local/lib/python3.6/site-packages/yaml/constructor.py", line 49, in get_single_data
    node = self.get_single_node()
  File "/usr/local/lib/python3.6/site-packages/yaml/composer.py", line 43, in get_single_node
    event.start_mark)
yaml.composer.ComposerError: expected a single document in the stream
  in "/opt/rules/all-secret-rules.yaml", line 2, column 1
but found another document
  in "/opt/rules/all-secret-rules.yaml", line 10, column 1

I have updated the documentation and provided examples of working secrets in the values.yaml.

Thanks!

[jertel]

Looks good. Will you please bump the chart.yaml version to 1.8.0?