Generate constraints file to pin deps in Docker images

Dockerfiles pin versions using constraints file

Pulling out strip_hashes and add --no-deps flag

tools/pip_install.py

@@ -19,6 +19,7 @@ import tempfile
 
 import merge_requirements as merge_module
 import readlink
+import strip_hashes
 
 
 def find_tools_path():
@@ -47,10 +48,7 @@ def certbot_normal_processing(tools_path, test_constraints):
     with open(certbot_requirements, 'r') as fd:
         data = fd.readlines()
     with open(test_constraints, 'w') as fd:
-        for line in data:
-            search = re.search(r'^(\S*==\S*).*$', line)
-            if search:
-                fd.write('{0}{1}'.format(search.group(1), os.linesep))
+        fd.write(strip_hashes.main(data))
 
 
 def merge_requirements(tools_path, requirements, test_constraints, all_constraints):