Multi volume support for glusterfs-hadoop

This enhancement will allow multiple gluster volumes accessible in a single hadoop configuration.  

These configuration changes to core-site.xml are required:
```xml
<property>
<name>fs.glusterfs.volumes</name>
<value>gv0,gv1</value>
</property>

<property>
<name>fs.glusterfs.volume.fuse.gv0</name>
<value>/mnt/gv0</value>
</property>

<property>
<name>fs.glusterfs.volume.fuse.gv1</name>
<value>/mnt/gv1</value>
</property>
```
The fs.glusterfs.volumes setting is a comma separated list of gluster volumes.  The default volume is considered the first volume in the list and can be accessed with no authority in the URI (ie. glusterfs:///path). 

For each volume, a fuse mount must be specified with fs.glusterfs.volume.fuse.<VOLNAME> setting.

The URI format for each volume is "glusterfs://volname/pathToFile".  The first volume in the list can be accessed without the volume name in URI.  On startup, the working-directory is set to "glusterfs:///user/" and will be used to relativize unqualified paths (unless set to something else during runtime).