build(ansible): add options to install grafana and dashboards

[lc525]

This PR adds an easier, automated way to deploy the MMS grafana dashboard used in various demos.

For an existing seldon core cluster, it is sufficient to run ansible-playbook playbooks/setup-ecosystem.yaml -e full_install=false -e install_grafana=true to add the grafana install.

For new ansible installs, the grafana dashboard is deployed by default. If may be disabled by passing install_grafana=false to the playbook

Details:

• ansible role to add grafana to seldon cluster
• allow configuring pre-loaded dashboards connected to prometheus data-source
• add seldon MMS grafana dashboard
• generate random password for grafana install

Which issue(s) this PR fixes:

• INFRA-992 (internal issue) Automate grafana dashboard deployments