Fix the empty string issue for pip

When read from requirement file, the grep not deal with
blank line, which cause pip wheel have '' issue.

Check logs in below for details:
http://logs.openstack.org/21/181821/1/check/check-requirements-integration-dsvm/3ce99ff/console.html#_2015-05-13_05_27_33_105

Change-Id: I558a9488f576738a44dd746589c04bd28d7cd332

tools/build_wheels.sh

@@ -40,7 +40,7 @@ then
     exit 1
 fi
 
-grep -v '^#' global-requirements.txt | while read req
+grep -v '^$\|#' global-requirements.txt | while read req
 do
     echo "Building $req"
     for v in $versions