libostree: Add new OstreeSysroot class

At the moment, just a container for a path, but we will start moving
admin functionality here.
Move Deployment and BootconfigParser into libostree

As part of moving admin functionality there.  While we are doing this,
rename OtConfigParser to OstreeBootConfig parser since it's a better
name.
libostree: Move sysroot initialization API here
libostree: Move a lot more sysroot API here

OstreeBootloader is temporarily public API.
libostree: Nearly complete move of API into OstreeSysroot

Move the deployment code too.
admin: Drop two uses of ostree_sysroot_get_path()

Moving towards it being a full library.
libostree: Make OstreeBootloader private

It was only temporarily public while functionality was being merged
down; that's done now.