Add a note to requirements.txt about Ansible dell network module bug 30350

requirements.txt

+# Ansible 2.3 has a bug (#30350) in the Dell network modules. Pull in a version
+# with the fix backported. It can be installed by uncommenting the following
+# line and commenting the one after.
+# -e git+https://github.com/stackhpc/ansible@issue-30350-2.3#egg=ansible-issue-30350-2.3 # GPLv3
 ansible<2.4.0 # GPLv3
 cliff>=2.5.0 # Apache
 netaddr!=0.7.16,>=0.7.13 # BSD