configurable secure argument for minio client

[xaniasd]

with this change it is possible to configure the secure argument when instantiating a minio client in the following 2 ways:

• use an S3_ENDPOINT with https scheme;
• set environment variable USE_SSL to "false"

The default value remains True.

Resolves #825

[ukclivecox]

We would need to check how this is being used by the Seldon Operator in the init container for this merged fix to be usable.

[xaniasd]

@cliveseldon I think setting S3_ENDPOINT with http:// should do the trick, I'll let you know once I've tried it