Add Rocketchat alert plugin v0.1.2

[fydrah]

Hi,

I created a rocketchat alert plugin for Cabot (https://github.com/ObjectifLibre/cabot-alert-rocketchat)

This PR:

• includes plugins installation on requirements-plugins
• update the default example env file
• add the 3 required env vars used to configure the plugin

regards,

[codecov[bot]]

Codecov Report

Merging #622 into master will increase coverage by 0.07%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #622      +/-   ##
==========================================
+ Coverage    80.9%   80.98%   +0.07%     
==========================================
  Files          46       45       -1     
  Lines        2954     2918      -36     
  Branches      179      177       -2     
==========================================
- Hits         2390     2363      -27     
+ Misses        505      497       -8     
+ Partials       59       58       -1

Impacted Files	Coverage Δ
cabot/urls.py	82.5% <0%> (-0.84%)	:arrow_down:
cabot/cabotapp/models/jenkins_check_plugin.py	64.91% <0%> (-0.61%)	:arrow_down:
cabot/templates/base.html	94.59% <0%> (-0.28%)	:arrow_down:
cabot/templates/base_public.html	99.2% <0%> (-0.02%)	:arrow_down:
cabot/templates/404.html
cabot/cabotapp/views.py	71.35% <0%> (+0.46%)	:arrow_up:

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