multi-user/multi-tenancy support for hadoop on glusterfs

This change adds support for multi-user scenario's to hadoop and glusterfs using POSIX ACL mechanism.

First enable ACLs for your gluster mount. Next pick & create a limited 'daemon user' for hadoop daemons to run under.  This user will be given read/write access to files on the DFS. Then start your hadoop daemons as this user.

Finally set  a core-site.xml configuration parameter:

  <property>
    <name>gluster.daemon.user</name>
    <value>hadoop</value>
  </property>

where the value is the super user picked earlier.

Finally, submit map/reduce jobs as a valid user on the system.  Standard POSIX permissions, file modes and acl rules/utilities work as usual.

Tested on 1.2 and 2.0.5-alpha.