[Tachyon-26] test UnderFileSystem.get() code path in Glusterfs Cluster.

Fix the dependency issue in gluster test profile;
Update gluster test instruction in glusterfs setup document.

Signed-off-by: rootfs <hchen@redhat.com>
yes, command line argument tachyon.underfs.glusterfs.mounts=xxx provides the mount points into unit test. Making it static or hard coded in unit test is a bit undeserable if you have a glusterfs on your host and it is mounted somewhere else.
I think the people who run gluster unit test are those who use glusterfs. The command line arg may not be the most convenient but it do serve the dual purposes well.