Add shade_pip_executable variable

Expose shade_pip_executable to allow user to specific version of
pip.

Change-Id: I2d8c5316cd5c891bda5fa3c3dc2de813c8a5e49b
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -21,6 +21,7 @@ shade_git_version: master
 shade_install_method: git
 
 shade_pip_name: shade
+# shade_pip_executable:
 # shade_pip_extra_args:
 # shade_pip_version:
 # shade_pip_virtualenv_python:

tasks/install/pip.yaml

@@ -15,6 +15,7 @@
 - name: Install shade using pip.
   become: yes
   pip:
+    executable: "{{ shade_pip_executable|default(omit) }}"
     extra_args: "{{ shade_pip_extra_args|default(omit) }}"
     name: "{{ shade_pip_name }}"
     version: "{{ shade_pip_version|default(omit) }}"