Added options to improve offline usage.

 * NOVA_ZIPBALL_URL can be set for a XS/XCP install to point to a
   non-standard zipball location.
 * PIP_USE_MIRRORS (default True) can be set to False to stop pip
   from attempting to use mirrors (useful for when using an offline
   eggproxy).

Change-Id: I01a9944fe4c13c8cc8323a703f598bc498491b73
diff --git a/tools/xen/install_os_domU.sh b/tools/xen/install_os_domU.sh
index 0bb6ac8..c78c6f2 100755
--- a/tools/xen/install_os_domU.sh
+++ b/tools/xen/install_os_domU.sh
@@ -57,8 +57,8 @@
 fi
 
 # get nova
-nova_zipball=$(echo $NOVA_REPO | sed "s:\.git$::;s:$:/zipball/$NOVA_BRANCH:g")
-wget $nova_zipball -O nova-zipball --no-check-certificate
+NOVA_ZIPBALL_URL=${NOVA_ZIPBALL_URL:-$(echo $NOVA_REPO | sed "s:\.git$::;s:$:/zipball/$NOVA_BRANCH:g")}
+wget $NOVA_ZIPBALL_URL -O nova-zipball --no-check-certificate
 unzip -o nova-zipball  -d ./nova
 
 # install xapi plugins