Change pip install method and add six to requirements

Minor changes to make Bifrost's test/installation sequence more
compatible with Centos7.  Attempt easy_install based install
for pip if it is missing and install six from pypi as it
is required by Ansible.

Change-Id: Ia583bcaa15239753b7f50afbae001aff0904efd3
Partial-Bug: 1463871

requirements.txt

@@ -5,3 +5,4 @@ pbr>=0.6,!=0.7,<1.0
 Babel>=1.3
 oslo.config>=1.11.0
 oslo.log>=1.2.0
+six>=1.9.0

scripts/env-setup.sh

@@ -6,7 +6,7 @@ if [ -x '/usr/bin/apt-get' ]; then
         sudo -H apt-get -y install git
     fi
     if ! $(pip -v &>/dev/null); then
-        sudo -H apt-get -y install python-pip
+        sudo easy_install pip
     fi
 elif [ -x '/usr/bin/yum' ]; then
     if ! $(git --version &>/dev/null); then