Add direct lustre api calls

Adds lustre api calls to scan OSTs for their sizes and a call to create a file with a given stripe and count.

The improvements over the existing methods are that the selection chooses emptier OSTs first, so balances the use of OSTs, and provides the ability to not use OSTs that are too full (so do not have space for the file with stripe 1 to be written, that causes an IO error) and to avoid trying to use OSTs that have been closed, so it is not possible to write to them (this happens if the OST has a fault or is closed by the administrator as the OST is very full and is likely to cause IO errors). When an OST is not writable the next OST is selected instead, so by choosing OSTs in some numeric order we can end up with a single OST taking the load for all the closed ones as well, that depends on whether the OSTs are also closed in some order (on COSMA this tends to be the case).