Create zuul_pip_executable

Allow the user to control the path to the pip executable.

Change-Id: I0622a8cf6ceb0b3fc2583dca106cecc759190207
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -68,6 +68,7 @@ zuul_git_version: feature/zuulv3
 zuul_install_method: git
 
 zuul_pip_name: zuul
+# zuul_pip_executable:
 # zuul_pip_extra_args:
 # zuul_pip_version:
 # zuul_pip_virtualenv_python

tasks/install/pip.yaml

@@ -15,6 +15,7 @@
 - name: Install zuul using pip.
   become: yes
   pip:
+    executable: "{{ zuul_pip_executable|default(omit) }}"
     extra_args: "{{ zuul_pip_extra_args|default(omit) }}"
     name: "{{ zuul_pip_name }}"
     version: "{{ zuul_pip_version|default(omit) }}"