Feat/Update grafana dashboards showing negative values

[slopezz]

• A few dashboards whose specific metrics tends to be 0, are showing negative values because Ymin=null (which means is auto).

  • On some cases where values are very high, Ymin=null (auto) is good, because it adapts automatically to each value, making simpler to check the evolution:
    • Good having Ymin=null (auto)
      
    • Instead of Ymin=0 (bad)
      
  • However, on some case where metrics tend to be 0, having Ymin=null (auto) means showing negative values on the panel, which makes no sense:
    
    • This PR fixes a few cases on memcached and postgres grafana dashboards where it is better to have Ymin=0

• In addition, it simplifies the mysql database example used to show how the exporter can be deployed, because it stopped working after last time it was checked.

• Alpha release v0.6.0-alpha.3 has been successfully tested in development cluster.

/kind bug
/priority important-soon
/assign

[3scale-robot]

LGTM label has been added.

Git tree hash: 8e32c1467f018ce298ddcc6a61f824c2e749f61e

[raelga]

/lgtm

[slopezz]

/approve

[3scale-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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