Clean-up before 2.0 release

1.9 includes new similar node_exporter+dockerd+cadvisor Grafana dashboards in addition to the existing Netdata+dockerd+cadvisor ones. This gives users a choice between their data sources for monitoring server nodes and containers within their clusters. This is in preparation for the swarmstack 2.0 release, which will include the installation of node_exporter via systemd onto nodes in addition to or in lieu of installing Netdata on each node under systemd. Recommendation is run both exporters (default now).

-- original commit comments --
preserve alertmanager silences into /alertmanager volume mount
rev caddy, grafana, prometheus
default https://github.com/clockworksoul/docker-gc-cron to not automa… …
enable netdata service after reboot
rev Portworx storage to 2.1.4
some bit-rot had set into this dashboard, switched to log() view on m… …
rev Grafana to 6.3.4 due to CVE-2019-15043
move to use grafana:latest tag as this follows their release versions… …
Updated Nodes Grafana dashboards, both for netdata+dockerd+cadvisor a… …
move to alertmanager:latest, updated node and container dashboards
new node_exporter + dockerd + cadvisor Nodes dashboard
update node and container dashboards with netdata or node_exporter me… …
rename a grafana panel
dashboard cleanup / sync
cleaning up dashboards
remove rebooted restriction when waiting for portworx storage on node… …
increase reboot timeout wait from 2 minutes to 3 before timing out
deprecate storage.tsdb.retention directive to prometheus