encrypt-file: roll back extended file stat

Change I8e7bc38c68c224795630b90a1b989098a7661491 switched the
"Validate input file" task to use a path stat, but when a list of
files is passed into encrypt_file this fails because the list is
treated as a string. Switch back to the more naive "is undefined"
check until it can be properly redone with a loop or similar and
appropriate tests added.

Change-Id: I83e665bc890aec42462776e0079457bb4506552f

roles/encrypt-file/tasks/main.yaml

@@ -1,12 +1,7 @@
-- name: Stat input file
-  stat:
-    path: '{{ encrypt_file }}'
-  register: _stat_result
-
 - name: Validate input file
   fail:
-    msg: '{{ encrypt_file }} : file does not exist'
-  when: not _stat_result.stat.exists
+    msg: 'Must define "encrypt_file"'
+  when: encrypt_file is undefined
 
 - name: Ensure gpg2 installed
   package: