ensure failure to inject user files results in startup error
ensure failure to inject user files results in startup error

This was the case in Essex but was inadvertantly changed in:
folsom-2-95-g0d166ca.

* nova/virt/disk/api.py: Refactor to allow specifying
mandatory injection items, that result in an exception
on failure to inject.
* nova/virt/libvirt/driver.py: Specify that user 'files'
are mandatory items and thus result in VM startup failure
unless injected successfully.
* nova/tests/test_virt_disk.py: A new test for the
separate warning and error cases.

Fixes bug: 1095744
Change-Id: Idab5c4294c1cb52098ce44a7aae957a44fb2674f

n0ano/ganttclient