Dnsrobocert

Changed
* Update Lexicon to 3.2.7 (add Aliyun, Azure DNS and GratisDNS providers)
* Update Certbot to 0.35.1 (various fixes)

Changed
* Certbot's work dir and logs dir are now set to be in the config dir `/etc/letsencrypt` (respectively under
`work` and `logs`). This allows to persist both certbot backups and logs.

Added
* First implementation of autorestart feature in a Docker cluster:
using environment variable `DOCKER_CLUSTER_PROVIDER (default: none)`, the container will use appropriate
service restart command to restart the service name specified in `autorestart-containers=...` directive.
For now, only Swarm is supported: support is triggered using `DOCKER_CLUSTER_PROVIDER=swarm`.

Changed
* Python libraries uses now the pip option --no-cache-dir to reduce container footprints.
* The autorestart and autocmd directives are now triggered upon certificate renewal as before, but also
upon first certificate issuance now.

Added
* Environment variables `CRON_TIME_STRING (default: "12 01,13 * * *")` and `TZ (default: UTC)` allow to control
at which frequency, and on which timezone the renewal cron job will be executed. By default it is twice a day,
at midday and midnight UTC.

Changed
* Update Lexicon to 3.2.5
* Update Certbot to 0.34.2

Added
* Environment variable `DEPLOY_HOOK` (default: _empty_) can be set to execute a shell command when a certificate
is created or renewed. This deploy command can be used for instance to do some post-process formatting on the
certificates before their deployment on a service requiring a specific certificate format.

Changed
* Update Lexicon to 3.2.4 (new providers: Netcup)

Added
* Environment variable `LETSENCRYPT_SKIP_REGISTER` (default: `false`) can be set to `true` and avoid the container
to try to register a new account against Let'sEncrypt servers: useful for instance if the container is already
attached to a letsencrypt configuration folder with an existing account.
* By modifying the `run.sh` script, the container now responds to interruption signals and proceed to shutdown by
itself, without the need from the Docker daemon to kill it.

Changed
* Update Lexicon to 3.2.1 (various fixes)
* Update Certbot to 0.33.1 (various fixes)