Add SSL support

[srilumpa]

Hi,

This is an implementation proposal that should fix #56. The code is compiling but I haven't tested it (I don't know how to include the built project into a fresh TheHive or Cortex build).

Sorry for having removed the for comprehension structure used to generate the xpackClient but I haven't found a proper solution to keep it with multiple independent options so I got back to a more classical way to handle to client setting.

This pull request adds the following features:

• Handle SSL transport between client and server
• Can set path to custom CA if not included in the java truststore
• Handle client authentication through certificate

Here is a sample configuration that should be supported from now on:

search {
  # Name of the index
  index = my_index
  # Name of the Elasticsearch cluster
  cluster = my_cluster
  # Address of the Elasticsearch instance
  host = ["127.0.0.1:9300"]
  # Scroll keepalive
  keepalive = 1m
  # Size of the page for scroll
  pagesize = 50
  # Number of shards
  nbshards = 5
  # Number of replicas
  nbreplicas = 1
  # Arbitrary settings
  settings {
    # Maximum number of nested fields
    mapping.nested_fields.limit = 100
  }
  # Username to log into the database
  username = application_user
  # password
  password = my_super_duper_password
  # SSL settings
  ssl {
    # Activate SSL support whhen connection to the database (default: false)
    enabled = true
    # Path to the custom CA certificate (default: will use the Java truststore)
    ca = /opt/my_app/ca.pem
    # Paths to the certificate and key used to log into the database if client certificate authentication is enabled on the Elastic cluster (default: none)
    certificate = /opt/my_app/client_cert.pem
    key = /opt/my_app/client_key.pem
  }
}

I hope this will fit your expectations. Don't hesitate to point me some improvements to be made if necessary.