Add writeup on how to upgrade a license file on a running cluster

[edbernier]

In the docs there are a couple of brief descriptions of license-key and license-path but there is no description of how to install a new license on a running cluster.

Do I need to stop and start the cluster after?
Do I simply replace my current JSON license file?
Do I use the same license file on all my nodes?
Do I need to update the meta nodes and the data nodes?
Is there a command I should issue to signal InfluxDB to reread the license file?

We should add a section in the docs on the process to go through and highlight the fact that you can do this on a running cluster with no downtime.

URL for relevant page? https://docs.influxdata.com/enterprise/v1.2/administration/configuration/

What products and version are you using? 1.2

Where did you look before opening the issue? 1.2 docs

[timhallinflux]

@edbernier -- added to 1.3 Enterprise docs.