Add dstat graph roles

Change-Id: Idc359579b307f5f156dfe5fb5aab5d99bdc46884

roles/run-dstat/README.rst

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+Run dstat
+
+Add this to a pre-run playbook to run ``dstat``.
+
+The role :zuul:role:`dstat-graph` may optionally be used to graph the
+resulting data.
+
+**Role Variables**
+
+.. zuul:rolevar:: dstat_data_path
+   :default: "{{ ansible_user_dir }}/zuul-output/logs/dstat.csv"
+
+   The path to the dstat data file.

roles/run-dstat/defaults/main.yaml

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+dstat_data_path: "{{ ansible_user_dir }}/zuul-output/logs/dstat.csv"

roles/run-dstat/tasks/main.yaml

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+- name: Install dstat
+  package:
+    name: dstat
+    state: present
+  become: true
+- name: Run dstat
+  shell: "dstat -tcmndrylpg --tcp --output {{ dstat_data_path }} >& /dev/null &"
+  args:
+    executable: /bin/bash