Custom Meta WAL Provider doesn't default to custom WAL Provider whose configuration value is outside the enums in Providers

Currently all the WAL Providers acceptable to hbase are specified in Providers enum of WALFactory.
This restricts the ability for custom Meta WAL Provider to default to the custom WAL Provider which is supplied by class name.

This issue fixes the bug by allowing the specification of new WAL Provider class name using the config "hbase.wal.provider".