master, tools: Add support for named goals

Add a possibility to set and get named goals using mfssetgoal/mfsgetgoal.

This commit also modifies test_moosefs_upgrade to use mfstools from
MooseFS before the master server is upgraded from mfs-1.6.27 to the
current version (new mfstools don't work with the old master server).
It also adds checking for backward compatibility of mfssetgoal and
mfsgetgoal to this test.

Change-Id: Ieae4ffa1c61ba736af27067d041b943b69bfefa0