Add a new unit test for LUInstanceMultiAlloc
Add a new unit test for LUInstanceMultiAlloc

This patch, extends the 'cmdlib.instance_unittest.py' with a new test
for instances multi allocations, in order to test an allocation of more
than one instances.

Signed-off-by: Dimitris Bliablias <356e9f69cc3964f845e374fffaf491fb57ee816f@skroutz.gr>
Signed-off-by: Lisa Velden <f95ae155456787aef400a49db463a62bbef84ead@google.com>
Reviewed-by: Lisa Velden <f95ae155456787aef400a49db463a62bbef84ead@google.com>

leshchevds/ganeti,bitemyapp/ganeti,onponomarev/ganeti,yiannist/ganeti,leshchevds/ganeti,andir/ganeti,ganeti/ganeti,leshchevds/ganeti,onponomarev/ganeti,ganeti/ganeti,ganeti/ganeti,andir/ganeti,yiannist/ganeti,mbakke/ganeti,yiannist/ganeti,andir/ganeti,bitemyapp/ganeti,mbakke/ganeti,mbakke/ganeti