Fixed some typos in docs

Will retry to download plugins once if the zip lib panics.

closes #4068

Pulled out the common code

refactor how template vars are updated. fixes #4283

Update data_source.md

Implemented GetDataSourceByName API

- When distributing a dashboard it is often not practical to have
"current" template variable option set. This commit ensures that
for dashboards with no current, it is assigned when the dashboard
loads (assuming refresh on load is set.)

Use promises to order the updates of variable options so that
parents are always updated before children.
This ensures that we only need to query the datasource once per
variable as variables that depend on other variables will only be
processed once their parent has been.
This commit also ensures that variable options are refreshed if
"refresh_on_load" is true even when query params are used to
set the variable seltion.

closes #4250