Feature #2494: The scheduler looks for LIMIT_MB in DS template

(cherry picked from commit 2d2f7c9d41b87b1cb9877c0dc5a116a42415ecd1)
Feature #2494: Change the meaning of limit_mb, add it to oned

Now limit_mb does not replace total_mb. It is the max amount of
storage that OpenNebula will use, regardless of the total disk
or the usage of the disk outside the DS dir

(cherry picked from commit 37af594b979bd5abf57cdd41b2abfc77c937dd6e)
Feature #2494: Pretty print DS limit_mb in CLI and Sunstone, add it to sunstone DS wizard

(cherry picked from commit e67a852af1c699bf213ff4b61e7cea8a92e1657f)