Refactor MinioAdminClient using HTTP client

[pbrw]

Attempt to solve these issues #829 #1243

• Implemented encryption/decryption of sensitive payload which is the hard part about using the HTTP Admin API
• Refactored code to reuse the request authentication functionality
• Implemented basic user management methods (user_add, user_remove, user_info, list_users)
• Wrote some tests

Now it should be straightforward to implement the rest of MinIO Admin API

[pbrw]

I rewrote it according to your requests. At the moment I commented all the MinioAdmin commands with mc and I will rewrite them with HTTP in subsequent commits

[balamurugana]

I rewrote it according to your requests. At the moment I commented all the MinioAdmin commands with mc and I will rewrite them with HTTP in subsequent commits

@pbrw I sent a PR pbrw#1 to your fork instead of lengthy review. Please review and merge it in your fork.

[balamurugana]

@pbrw I sent you PR pbrw#3 and reverted prometheus_create() addition. Few APIs are removed and site replication APIs are added. I also increased minor version to indicate backward compatibility.

[balamurugana]

@pbrw Please fix CI failure

[jessebot]

how do you specify credentials now? Previously MinioAdmin took an alias.