Add --dashboard-cipher-suites for dashboard TLS configuration

[echlebek]

Spec

Add configuration to sensu-backend that allows users to specify a comma-separate list of cipher suites for the dashboard's TLS server.

If the option is not specified, the server should use the default ciphers that are configured in https://github.com/sensu/sensu-go/blob/master/api/core/v2/tls.go.

Testing

Test the service with https://github.com/drwetter/testssl.sh with a known-good and a known-bad set of ciphers to ensure that the feature is working as expected.