rpmostreepayload: Handle /var as a user-specified mountpoint

See: ostreedev/ostree#855

Basically, if `/var` is in the user specified mounts, don't override
it with the "default ostree" `var`.

This allows people to easily keep all of their local state (apart from `/etc`)
in a single partition with potentially different quotas etc.

pyanaconda/packaging/rpmostreepayload.py

@@ -221,11 +221,14 @@ def prepareMountTargets(self, storage):
 
         # Set up bind mounts as if we've booted the target system, so
         # that %post script work inside the target.
-        self._binds = [(varroot, iutil.getSysroot() + '/var'),
-                       (iutil.getSysroot() + '/usr', None),
+        self._binds = [(iutil.getSysroot() + '/usr', None),
                        (iutil.getTargetPhysicalRoot(), iutil.getSysroot() + "/sysroot"),
                        (iutil.getTargetPhysicalRoot() + "/boot", iutil.getSysroot() + "/boot")]
 
+        # https://github.com/ostreedev/ostree/issues/855
+        if storage.mountpoints.get("/var") is None:
+            binds.append((varroot, iutil.getSysroot() + '/var'))
+
         # Bind mount filesystems from /mnt/sysimage to the ostree root; perhaps
         # in the future consider `mount --move` to make the output of `findmnt`
         # not induce blindness.