Pull credentials from environment variables rather than expecting them to be hard-coded in config file

EnvironmentVariableSecurityCredentialsProvider.scala

import org.apache.ignite.plugin.security.{SecurityCredentials, SecurityCredentialsProvider}

class EnvironmentVariableSecurityCredentialsProvider extends SecurityCredentialsProvider {
  override def credentials(): SecurityCredentials = new SecurityCredentials(sys.env("IGNITE_USERNAME"), sys.env("IGNITE_PASSWORD"))
}