[WIP] Implement CEPH OSD topology

[cristicalin]

This PR adds a new metric called ceph_osd_exists which is computed from the ceph osd tree output.
This metric has a topology label which is composed of the tree topology (not sorted). This sort of implements the requirement from #40

The resulting metric is a bit messy at the moment and needs metric_relabel_configs on the prometheus side to be cleanly stored.

The idea behind this is to have a lookup metric we can use with the label_values function to extract templated variables for use by grafana.

Here is my job definition with an example metric_relabel_config:

  - job_name: 'ceph-exporter'
    static_configs:
      - targets: ['172.18.0.2:9128']
        labels:
          alias: ceph-exporter
    metric_relabel_configs:
      - source_labels: [topology]
        regex:         '.*,root=([\w\-]+),.*'
        target_label:  'root'
        replacement:   '$1'
      - source_labels: [topology]
        regex:         '.*,room=([\w\-]+),.*'
        target_label:  'room'
        replacement:   '$1'
      - source_labels: [topology]
        regex:         '.*,host=([\w\-]+),.*'
        target_label:  'host'
        replacement:   '$1'
      - regex: 'topology'
        action: labeldrop

I'm currently asking for some feedback on the approach thus the [WIP] in the name.

[coveralls]

Coverage decreased (-1.8%) to 85.323% when pulling 71736a5 on cristicalin:add_osd_tree into ac79cf7 on digitalocean:master.

[coveralls]

Coverage decreased (-1.8%) to 85.323% when pulling 0bc08b3 on cristicalin:add_osd_tree into ac79cf7 on digitalocean:master.

[coveralls]

Coverage decreased (-2.0%) to 85.115% when pulling 5735461 on cristicalin:add_osd_tree into ac79cf7 on digitalocean:master.

[neurodrone]

Let's just name this osd_tree. That way it will be easier to understand what it means. We can also add more labels to it in future if need be.

[neurodrone]

Would it be possible to throw a quick test for collectOSDTree()? There are examples in osd_test.go which should help provide a sample.

[coveralls]

Coverage decreased (-2.0%) to 85.115% when pulling 2e50ab7 on cristicalin:add_osd_tree into ac79cf7 on digitalocean:master.

[coveralls]

Coverage decreased (-1.9%) to 85.218% when pulling 2de1cfe on cristicalin:add_osd_tree into c41f6ae on digitalocean:master.

[coveralls]

Coverage decreased (-1.9%) to 85.209% when pulling 40433e5 on cristicalin:add_osd_tree into c41f6ae on digitalocean:master.

[coveralls]

Coverage decreased (-2.5%) to 84.552% when pulling 7454d50 on cristicalin:add_osd_tree into c41f6ae on digitalocean:master.

[cristicalin]

I changed the approach to fetch the topology in the beginning of the run.
The compiled topology is then used for each osd specific metric.
The result of this approach is that OSD metrics will be duplicated in the output if the crushmap has multiple overlapping roots.
This also breaks the tests so still WIP.

Again comments are welcome to validate the approach before I ask for a merge.