use secrets for certificates

[CanisLupusLupus]

This PR resolves issue #21.

Certificates are projected in containers using kubernets secrets and are mounted in /etc/icinga2-pki/<feature>/ directory. On enabling TLS/SSL in feature all certificates are required (i.e. ca, cert and key).

Sample configuration for elasticsearch:

icinga2:
  features:
    elasticsearch:
      enabled: true
      secretName: icinga-elasticsearch-secret
      enable_tls: true
      caSecretKey: ca
      certSecretKey: cert
      keySecretKey: key

Besides resolving issue #21 there are some more related changes:

• I've renamed .Values.global.redis.password to .Values.features.icingadb.password.value and enabled configuration via value/secret (I've overloked this sensible value in PR #17)
• default value for .Values.features.influxdb.service_template is updated (closing }) was missing

[CanisLupusLupus]

I agree, it's better design to have separate secrets (and it also makes charts more flexible and easier to use...).

I'll go ahead and refactor the PR in coming days.

[CanisLupusLupus]

I've refactored the PR to use dedicated credSecret/tlsSecret instead of secretName.

[mocdaniel]

LGTM!