Try to fix broken stestr command discovery

When processing subunit streams we attempt to discover where the
(s)testr commands are located. For some reason the output of our scripts
that do this very occasionally emit a newline before the command path.
When this happens we were using the blank line as the command path which
fails as that empty command results in our command arguments being
interpreted as the command path.

Attempt to address this by trimming the stdout instead of taking the
first line. This should remove the leading and trailing newlines and
give us only the command itself.

Change-Id: Id651e019cf3d0b7ab37fdf9df04be249ea7f7af6

roles/fetch-subunit-output/tasks/find-subunit-html.yaml

@@ -6,9 +6,12 @@
 - name: Define subunit_html_command fact
   when:
     - find_subunit2html_output.rc == 0
-    - find_subunit2html_output.stdout_lines
+    - find_subunit2html_output.stdout
   set_fact:
-    subunit_html_command: "{{ find_subunit2html_output.stdout_lines[0] }}"
+    # We use the trim filter here on all stdout because for some reason
+    # we occasionally get the command listed with a blank line prefix.
+    # This should clean that up.
+    subunit_html_command: "{{ find_subunit2html_output.stdout | trim }}"
 
 - name: Emit a debug line so we can search logstash for fallback hits
   debug:

roles/fetch-subunit-output/tasks/main.yaml

@@ -49,9 +49,9 @@
 
 - when:
     - testr_command.rc == 0
-    - testr_command.stdout_lines
+    - testr_command.stdout
   # Here we run steps that only apply when there is a valid subunity stream.
-  # This is indicated through testr_command.stdout_lines content.
+  # This is indicated through testr_command.stdout content.
   block:
     # The usage an independent target file instead of sending the output
     # to zuul_work_dir prevents issues related to zuul_work_dir being
@@ -65,7 +65,10 @@
 
     - name: Generate subunit file
       shell:
-        cmd: "{{ testr_command.stdout_lines[0] }} last --subunit >>{{ temp_subunit_file.path }}"
+        # We use the trim filter here on all stdout because for some reason
+        # we occasionally get the command listed with a blank line prefix.
+        # This should clean that up.
+        cmd: "{{ testr_command.stdout | trim }} last --subunit >>{{ temp_subunit_file.path }}"
         chdir: "{{ zj_item }}"
       loop: "{{ all_subunit_dirs }}"
       loop_control: