dell-bootstrap: use distro policy for swap (Fixes: #82) (LP: #1842325)

It's much better than what we were adopting (needlessly wasting space)
> Secondly, the sizing of swapfiles is very different. It is no more than 5% of free disk space or 2GiB, whichever is lower.

See https://blog.surgut.co.uk/2016/12/swapfiles-by-default-in-ubuntu.html
for more details.

ubiquity/dell-bootstrap.py

@@ -364,12 +364,8 @@ def test_swap(self):
         release = lsb_release.get_distro_information()
 
         #starting with 17.04, we replace the whole swap partition to swap file
+        #use distro policy to determine what to do.
         if float(release["RELEASE"]) >= 17.04:
-            try:
-                self.db.set('partman-swapfile/percentage', '50')
-                self.db.set('partman-swapfile/size', self.mem * 2048)
-            except debconf.DebconfError as err:
-                self.log(str(err))
             return True
 
         if (self.mem >= 32 or self.disk_size <= 64):