allow disabling file injection completely

With the libvirt_injection_partition config set to -2,
file injection is avoided completely.

This can be required if you want to make user specified files
available from the metadata server (and config drive is not enabled),
then you would want to avoid injection completely for performance reasons,
but also to avoid boot failure if the injection itself failed.

Flags: DocImpact
Change-Id: I604f4189114515c70c58cf183622542c4633e44a
Merge "allow disabling file injection completely"