Mount admin certificate and key

Description

securityadmin.sh script needs some admin certificate and it key to make security changes in cluster.

Proposal

Add two variables with admin certificate and key and mount them to containers as admin.pem and admin-key.pem in /usr/share/elasticsearch/config

Links / references

• https://opendistro.github.io/for-elasticsearch-docs/docs/security-configuration/security-admin/#basic-usage
• https://opendistro.github.io/for-elasticsearch-docs/docs/install/docker-security/