Default mount table name used by ViewFileSystem should be configurable

Currently, if no authority is provided and the scheme of the Path is doesn't match the scheme of the fs.defaultFS , the mount table used by ViewFileSystem to resolve this path is default. 

This breaks accesses to path like hdfs:///foo/bar (without any authority) when the following configurations are used:
(1) fs.defaultFS = viewfs://clustername/ 
(2) fs.hdfs.impl = org.apache.hadoop.fs.viewfs.ViewFileSystemOverloadScheme

This JIRA proposes to add a new configuration fs.viewfs.mounttable.default.name.key which is used to get the name of the cluster/mount table when the authority is missing in cases like the above. If not set, the string default will be used as today.