Disable KMS Endpoints and Metrics Tabs if using KMS Secret Key

[cesnietor]

fixes: #3192

In Web UI, when configuring minio setting only the MINIO_KMS_SECRET_KEY, the Encryption section tries to perform /metrics, /apis and /version API requests but since the static key has no version nor metrics nor list apis, they return 500 error.

This change disables the tabs and only makes the requests when MINIO_KMS_SECRET_KEY is not set.

If KMS Secret Key not set and using KES

If KMS Secret Key set:

Test Steps:

Using Console from branch in MinIO Build:

1. Set KES like defined in docs

export MINIO_KMS_KES_ENDPOINT=https://play.min.io:7373    
export MINIO_KMS_KES_KEY_FILE=root.key
export MINIO_KMS_KES_CERT_FILE=root.cert
export MINIO_KMS_KES_KEY_NAME=my-minio-sse-kms-key
./minio server ~/Data

2. Go to Monitoring/Encryption (Metrics and APIs should be enabled and corresponding APIs need to be requested)

//// Setting only MINIO_KMS_SECRET_KEY

1. Set env like:

export MINIO_KMS_SECRET_KEY=my-minio-key:OSMM+vkKUTCvQs9YL/CVMIMt43HFhkUpqJxTmGl6rYw= ./minio server ~/Data

2. Go to Monitoring/Encryption (Metrics and APIs should be disabled and no extra callas apart from kms/status should be done)