Gitlab v15 has removed the "coverage" setting from the project settings,
it has to be added in gitlab ci job.

Badges needed to be updated since we switched from `master` to `main`
default branch.

This updates CI/CD pipeline to install latest dnspython on Debian Stretch
with pip3.

It also add cobertura report to view coverage status on Gitlab Merge
Request.

Some ACME HTTP requests missed the User-Agent (to get Nonce).

Centralize default configurations for acme_dns_python.

*BREAKING CHANGE* the `[DNS]` configuration section is completely optional and
modified: `Zone`, `Host` and `Port` have been removed and `NameServer` has been
added. See below for more information.

Before this version, `acme-dns-tiny` used the `Host` configuration as
`DNS` nameserver to send `DNS` resource record requests *AND* to install `DNS`
challenges.

Now, `acme-dns-tiny` uses by default the system DNS nameserver
to send `DNS` resource record requests.

You can change this behavior with the new `NameServer` settings which defines
a list of name server by IP addresses (separated by comma).

Now, acme-dns-tiny will install `DNS` resource records required
for the `ACME`challenges on the `DNS` zone defined by the `SOA` and `NS`
resource records for *each domain* defined on the certificate signing request
(the `CSR` file).

As said above, for these two `DNS` requests (`SOA` and `NS` resource records),
`acme-dns-tiny` will use by default the system `DNS` server or the one
configured with `NameServer`.

With `SOA` and `NS` `DNS` resource records, `acme-dns-tiny` is able to retrieve
zone name and authoritative name server automatically.
That's why `Zone` and `Host` configuration have been removed.
With these records, acme-dns-tiny can retrieve zone name and authoritative name
server list automatically.

In detail:
  * to resolve zone name acme-dns-tiny uses the dnspython built-in function
  * to lookup for master authoritative name servers, acme-dns-tiny queries
  SOA and NS resource records as described in the [RFC 2136](https://datatracker.ietf.org/doc/html/rfc2136#section-4).

ensure the nonce is used only once.

So, if this header is not a valid float number, we can
fallback to the default number of seconds.

Indeed the Request module returns a JSON object and not a python
dictionary when we use Request.repsone.json().

pebble is a Let's Encrypt project providing a simplified ACME
server implementation to run client tests.

Previously we set the string with IP as the list of nameservers.
So, for example, if IP were '172.16.1.1', the list of servers were
['1', '7', '2', '.', '1', '.', '1'] which was clearly falsy.

Now, it's correctly set as ['172.16.1.1'].

The POST-as-GET method described by the RFC requires to set empty
string as payload and not empty object.

If a DNS exception were raised during the loop, the dns_update message
were destroyed. On the next iteration of the loop, exception were raised
because of the NoneType instead of really trying next DNS server.

Thanks to Sebastien K. for the report and the debug !

acme_dns_tiny: skip validated authorization, add more information about authorization status and raise error if no dns-challenge has been found

Indeed, dnspython has only documented usage of IP address and not domain name.

Fix #11 (compatibility with dnspython 2.0)