deploy: Ensure that we can deploy using only /usr/lib/ostree-boot

rpm-ostree at least has the option to generate a tree with just that
instead of /boot, but while we were enumerating the latter, we'd still
return paths from /boot.

https://bugzilla.gnome.org/show_bug.cgi?id=740947
libotutil: Add ot_keyfile_copy_group()

Copies all the keys of a group from one GKeyFile to another.
repo: Add an internal struct to manage remotes

OstreeRemote is a reference-counted struct that encompasses data about a
remote, whether read from a configuration file or created explicitly via
ostree_repo_remote_add().

OstreeRemotes are held in an internal table indexed by remote name.
This solves some problems caused by merging system-wide remote data into
the OstreeRepo's internal config key file.

Also fixes https://bugzilla.gnome.org/show_bug.cgi?id=740911
repo: Add ostree_repo_remote_get_url()

Peeking at remote details by way of ostree_repo_copy_config() doesn't
work anymore.