Constraint Pandas and Numpy for gnocchi

The repo build is building pandas version 0.21.0rc1,
incompatible with gnocchi and numpy. This comes because gnocchi
isn't upper constrained, and the version to be installed is the
latest one available (due to a >= requirement).

We should instead constraint both pip packages to make possible
to install gnocchi.

Change-Id: If0e2cbebdbdebe7c71851d532b5f8629334b1aaf
Closes-Bug: #1724538

defaults/main.yml

@@ -168,6 +168,8 @@ gnocchi_pip_packages:
   - gnocchiclient
   - python-memcached
   - pycrypto
+  - pandas==0.20.3
+  - numpy==1.12.0
 
 # This variable is used by the repo_build process to determine
 # which host group to check for members of before building the