Add option to apply security groups to domain

[bgottlob]

what

• Adds capability to attach existing security groups to ElasticSearch domain by setting var.create_security_group to false
• Preserves default behavior of creating a new security group with one rule for each item in var.security_groups

why

• Some users expect var.security_groups to be a list of security groups to be directly attached to the ElasticSearch domain

references

• Resolves #62