Pin Flask to avoid installing 0.12.3

0.12.3 was released April 26th with a broken wheel package [1], 0.12.4
will be released momentarily.

[1]: pallets/flask#2728

Change-Id: I9bc9cad78423898ebbe58f2270d3f2407409448f
Signed-off-by: Sorin Sbarnea <ssbarnea@redhat.com>

requirements.txt

@@ -2,7 +2,7 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 
 jinja2>=2.9
 ansible>=2.2.0.0,!=2.4.0.0
-Flask>=0.11,<0.13
+Flask>=0.11,<0.13,!=0.12.3
 Flask-SQLAlchemy
 Flask-Migrate
 Flask-Script