Update env vars to meaningful names

[nitisht]

• Changed environment var S3_ADDRESS to SERVER_ENDPOINT
• Changed environment var S3_SECURE to ENABLE_HTTPS

[krishnasrinivas]

I think it is better to prefix the env vars by MINIO_TEST_

MINIO_TEST_ENDPOINT
MINIO_TEST_ACCESS_KEY
MINIO_TEST_SECRET_KEY
MINIO_TEST_ENABLE_HTTPS

[harshavardhana]

MINIO_TEST_ENDPOINT
MINIO_TEST_ACCESS_KEY
MINIO_TEST_SECRET_KEY
MINIO_TEST_ENABLE_HTTPS

Perhaps we can use these consistently across all libraries.. 👍

[krisis]

Whatever name we pick for environment variables it is best defined as golang constants. I.e,

const (
serverEndpoint = "SERVER_ENDPOINT",
...
)

[nitisht]

Minio SDKs will run against a target that is S3 compatible. So, I thought adding MINIO/S3 will be redundant.

@krisis yes, that will be better. I will do it

[nitisht]

Added constants

const (
	serverEndpoint = "SERVER_ENDPOINT"
	accessKey      = "ACCESS_KEY"
	secretKey      = "SECRET_KEY"
	enableSecurity = "ENABLE_HTTPS"
)