Use variant UID for subvariant fallback

The name can contains spaces, which would potentially cause problems.
Also this is not consistent with how it works in other phases.

Relates: https://pagure.io/pungi-fedora/pull-request/354
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/phases/buildinstall.py

@@ -348,7 +348,7 @@ def link_boot_iso(compose, arch, variant, can_fail):
     img.disc_number = 1
     img.disc_count = 1
     img.bootable = True
-    img.subvariant = variant.name
+    img.subvariant = variant.uid
     img.implant_md5 = implant_md5
     setattr(img, 'can_fail', can_fail)
     setattr(img, 'deliverable', 'buildinstall')

pungi/phases/ostree_installer.py

@@ -120,7 +120,7 @@ class OstreeInstallerThread(WorkerThread):
         img.disc_number = 1
         img.disc_count = 1
         img.bootable = True
-        img.subvariant = variant.name
+        img.subvariant = variant.uid
         img.implant_md5 = implant_md5
         setattr(img, 'can_fail', self.can_fail)
         setattr(img, 'deliverable', 'ostree-installer')