Add a notion of "physical" sysroot, use for remote writing

Add a notion of "physical" sysroot, use for remote writing

(Note this PR was reverted in <https://github.com/ostreedev/ostree/pull/902>;
 this version should be better)

Using `${sysroot}` to mean the physical storage root: We don't want to write to
`${sysroot}/etc/ostree/remotes.d`, since nothing will read it, and really
`${sysroot}` should just have `/ostree` (ideally). Today the Anaconda rpmostree
code ends up writing there. Fix this by adding a notion of "physical" sysroot.
We determine whether the path is physical by checking for `/sysroot`, which
exists in deployment roots (and there shouldn't be a `${sysroot}/sysroot`).

In order to unit test this, I added a `--sysroot` argument to `remote add`.
However, doing this better would require reworking the command line parsing for
the `remote` argument to support specifying `--repo` or `--sysroot`, and I
didn't quite want to do that yet in this patch.

This second iteration of this patch fixes the bug we hit the first time;
embarassingly enough I broke `ostree remote list` finding system remotes.
The fix is to have `ostree_repo_open()` figure out whether it's the same
as `/ostree/repo` for now.

Down the line...we might consider having the `ostree remote` command line itself
instatiate an `OstreeSysroot` by default, but this maximizes compatibility; we
just have to pay a small cost that `ostree` usage outside of that case like
`ostree static-delta` in a releng Jenkins job or whatever will do this `stat()`
too.

Closes: https://github.com/ostreedev/ostree/issues/892
Closes: #1008
Approved by: mbarnes