Pin the base distro release version in the master branch(Newton).
Only one distro version is supported. The supported versions are:

* CentOS: 7
* RedHat: 7
* OracleLinux: 7
* Debian: 8
* Ubuntu: 14.04

NOTE: Kolla will move to Ubuntu 16.04 in the Newton cycle

DocImpact
TrivialFix

Change-Id: I32c0edbee4de14c8a0cc7c961d566e84d70f19a4