ostree-sysroot: make simple_write_deployment smarter

ostree-sysroot: make simple_write_deployment smarter

This is a follow-up to https://github.com/ostreedev/ostree/pull/1097.
We make simple_write_deployment smart enough so that it can be used for
rpm-ostree's purposes. This is mostly an upstreaming of logic that
already existed there.

Notably we correctly append NOT_DEFAULT deployments *after* the booted
deployment and we now support RETAIN_PENDING and RETAIN_ROLLBACK flags
to have more granularity on deployment pruning.

Expose these new flags on the CLI using new options (as well as expose
the previously existing NOT_DEFAULT flag as --not-as-default).

I couldn't add tests for --retain-pending because the merge deployment
is always the topmost one. Though I did check that it worked in a VM.

Closes: #1110
Approved by: cgwalters