Reduce number of envs and options from command line. (#3230)

Ref #3229

After review with @abperiasamy we decided to remove all the unnecessary options

- MINIO_BROWSER (Implemented as a security feature but now deemed obsolete
  since even if blocking access to MINIO_BROWSER, s3 API port is open)
- MINIO_CACHE_EXPIRY (Defaults to 72h)
- MINIO_MAXCONN (No one used this option and we don't test this)
- MINIO_ENABLE_FSMETA (Enable FSMETA all the time)

Remove --ignore-disks option - this option was implemented when XL layer
 would initialize the backend disks and heal them automatically to disallow
 XL accidentally using the root partition itself this option was introduced.

This behavior has been changed XL no longer automatically initializes
`format.json`  a HEAL is controlled activity, so ignore-disks is not
useful anymore. This change also addresses the problems of our documentation
going forward and keeps things simple. This patch brings in reduction of
options and defaulting them to a valid known inputs.  This patch also
serves as a guideline of limiting many ways to do the same thing.

cmd/xl-v1.go

@@ -18,7 +18,9 @@ package cmd
 
 import (
 	"fmt"
+	"os"
 	"sort"
+	"strings"
 
 	"github.com/minio/minio/pkg/disk"
 	"github.com/minio/minio/pkg/objcache"
@@ -129,6 +131,9 @@ func newXLObjects(storageDisks []StorageAPI) (ObjectLayer, error) {
 	// Initialize list pool.
 	listPool := newTreeWalkPool(globalLookupTimeout)
 
+	// Check if object cache is disabled.
+	objCacheDisabled := strings.EqualFold(os.Getenv("_MINIO_CACHE"), "off")
+
 	// Initialize xl objects.
 	xl := xlObjects{
 		storageDisks:    newStorageDisks,
@@ -136,7 +141,7 @@ func newXLObjects(storageDisks []StorageAPI) (ObjectLayer, error) {
 		parityBlocks:    parityBlocks,
 		listPool:        listPool,
 		objCache:        objCache,
-		objCacheEnabled: globalMaxCacheSize > 0,
+		objCacheEnabled: !objCacheDisabled,
 	}
 
 	// Figure out read and write quorum based on number of storage disks.