Add amqps protocol suppport for rabbitmq export

Fix grafana json

Fix grammar in sensors documentation

Reflect "used percent" user disk space for [fs] alert

Needed to handle case where no protocol was specified in configuration. So added in an if clause to check if the protocol was present

The current json only works if the prefix is localhost.
Signed-off-by: NPaulo Flabiano Smorigo <pfsmorigo@gmail.com>

Depends of PR https://github.com/nicolargo/glances/pull/1680 to be merged

`psutill` [says the following](https://github.com/giampaolo/psutil/blob/master/psutil/_psposix.py):

>  Note: UNIX usually reserves 5% disk space which is not accessible
    by user. In this function "total" and "used" values reflect the
    total and used disk space whereas "free" and "percent" represent
    the "free" and "used percent" user disk space.

To fix the alert value that has been notified in this issue https://github.com/nicolargo/glances/issues/1658 where the alert has not been fixed.
The issue https://github.com/nicolargo/glances/issues/644 has fixed it one way, this PR is completing the fix for the alert feature of the `fs` plugin.

Fixed formatting on FS example

Update master dockerfile for Ubuntu 20.04

Not need for python-dev anymore

https://github.com/termux/termux-packages/issues/4151

Asymetric, cluttered configuration: Some options only accessible via CLI or keyboard or config file #1650 (correct IRQ)

Merge branch 'develop' of github.com:nicolargo/glances into develop

Add python 3.7 and 3.8 into CI