Allow setting puppet_version explicitly

If we set puppet_version as an inventory variable, then we can drive
behavior from that variable, including updating puppet. Add the ability
to set it here. A followup change in system-config will start setting
it, but starting off it should be a no-op.

Change-Id: I773ba3a2137b62ae2b3440dd02b3cdfb2099a62d

tasks/main.yaml

@@ -26,10 +26,12 @@
   shell:
     cmd: "PATH=$PATH:/opt/puppetlabs/bin puppet --version | cut -d '.' -f 1"
   register: puppet_version
+  when: puppet_version is not defined
 
 - name: Set puppet version fact
   set_fact:
     puppet_version: "{{ puppet_version.stdout }}"
+  when: puppet_version is not defined
 
 - name: Sanity check puppet version
   fail: "Unsupported puppet version {{ puppet_version }}"