lib/deploy: Fix staged deployments with no kargs

lib/deploy: Fix staged deployments with no kargs

Testing out the staged API with rpm-ostree, ostree-prepare-root.service
in the initramfs was failing.  Turned out that was because we didn't
have a `root=` kernel argument.  Which was because we didn't have
any kernel arguments at all except `ostree=`.

That in turn was because we weren't loading the bootloader config
from the merge deployment.

The serialized deployment data holds the unique identity of
(osname, checksum, deployserial) - look for the real merge deployment
in our deployment list which has the bootloader arguments we need.

This issue was entirely masked by the `ostree admin deploy` command
which itself explicitly loads the merge deployment's kernel arguments
in every case - it never passes the `NULL` default down.  A followup
patch will fix that.

Closes: #1558
Approved by: jlebon