Add name of the system pip package into defaults

On Amazon Linux, installing `python-pip` acutally installs the
`python26-pip` package, so anything running in Python 2.7 won't
find the dependencies installed with that `pip`.

This allows the package name to be customised, depending on the OS
and Python versions.

defaults/main.yml

@@ -1,2 +1,3 @@
 ---
+pip_package_name: python-pip
 pip_install_packages: []

tasks/main.yml

@@ -1,6 +1,8 @@
 ---
 - name: Ensure Pip is installed.
-  package: name=python-pip state=present
+  package:
+    name: "{{pip_package_name}}"
+    state: present
 
 - name: Ensure pip_install_packages are installed.
   pip: