elements: pip-and-virtualenv: Add python-xml dependency

SUSE packages the 'xml' python module as a separate package so make
sure it's pulled in before we attempt to install the pip module
since the latter depends on it. Fixes the following problem when
building with the opensuse-minimal and pip-and-virtualenv elements:

Traceback (most recent call last):
  File "/tmp/get-pip.py", line 19177, in <module>
    main()
  File "/tmp/get-pip.py", line 194, in main
    bootstrap(tmpdir=tmpdir)
  File "/tmp/get-pip.py", line 82, in bootstrap
    import pip
  File "/tmp/tmpOiESjX/pip.zip/pip/__init__.py", line 16, in <module>
  File "/tmp/tmpOiESjX/pip.zip/pip/vcs/subversion.py", line 9, in <module>
  File "/tmp/tmpOiESjX/pip.zip/pip/index.py", line 32, in <module>
  File "/tmp/tmpOiESjX/pip.zip/pip/_vendor/html5lib/__init__.py", line 16, in <module>
  File "/tmp/tmpOiESjX/pip.zip/pip/_vendor/html5lib/html5parser.py", line 6, in <module>
  File "/tmp/tmpOiESjX/pip.zip/pip/_vendor/html5lib/inputstream.py", line 10, in <module>
  File "/tmp/tmpOiESjX/pip.zip/pip/_vendor/html5lib/utils.py", line 10, in <module>
ImportError: No module named xml.etree.ElementTree

Change-Id: I1bec12dfcde05fb07f41bcec994148c3eacbb287

elements/pip-and-virtualenv/package-installs.yaml

@@ -1,3 +1,7 @@
+# This package is only valid for SUSE and we need to pull it in
+# early enough because pip depends on it.
+python-xml:
+  phase: pre-install.d
 python-pip:
   installtype: package
 python-virtualenv:

elements/pip-and-virtualenv/pkg-map

@@ -3,6 +3,12 @@
     "gentoo": {
       "python-pip": "dev-python/pip",
       "python-virtualenv": "dev-python/virtualenv"
+    },
+    "suse" : {
+      "python-xml": "python-xml"
     }
+  },
+  "default": {
+    "python-xml": ""
   }
 }