Create nodepool_pip_executable variable

Allow the user to override the location of the pip executable.

Change-Id: I47a13c168abcf35eb791ac602532fcea3f0e2a1e
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -63,6 +63,7 @@ nodepool_git_version: feature/zuulv3
 nodepool_install_method: git
 
 nodepool_pip_name: nodepool
+# nodepool_pip_executable:
 # nodepool_pip_extra_args:
 # nodepool_pip_version:
 # nodepool_pip_virtualenv_python:

tasks/install/pip.yaml

@@ -15,6 +15,7 @@
 - name: Install nodepool using pip.
   become: yes
   pip:
+    executable: "{{ nodepool_pip_executable|default(omit) }}"
     extra_args: "{{ nodepool_pip_extra_args|default(omit) }}"
     name: "{{ nodepool_pip_name }}"
     version: "{{ nodepool_pip_version|default(omit) }}"