Preconfigured Docker Registry credentials on the server

[saitho]

Allow to configure the credentials for Docker registry before hand on the server using pass.

• Make "password" field in container.registries of project definition file optional
• If none is given, assume the password for combination of registry and username is already configured (docker login [registry])
• If password is given, set the password as usual (via docker login [registry] [username] [password])
• Add server configuration setting to disallow manipulations of password storage => all registry credentials need to be configured beforehand; project deployment is not allowed to add or override credentials
  • see Merge refs/heads/task/serverconfig #180