icinga: grant access to private puppet directories

The nagios user can't access private puppet file and is unable to retrieve the current activation status. The user is declared on the main icinga2 class to also add the group on the icinga master (pergamon)

Related to T2566

Test Plan

diff origin/production/pergamon.softwareheritage.org current/pergamon.softwareheritage.org
*******************************************
+ Group[nagios] =>
   parameters =>
     "system": true
*******************************************
+ User[nagios] =>
   parameters =>
     "gid": "nagios",
     "groups": [
       "puppet"
     ],
     "home": "/var/lib/nagios",
     "shell": "/usr/sbin/nologin",
     "system": true
*******************************************
*** End octocatalog-diff on pergamon.softwareheritage.org

diff origin/production/moma.softwareheritage.org current/moma.softwareheritage.org
*******************************************
+ Group[nagios] =>
   parameters =>
     "system": true
*******************************************
+ User[nagios] =>
   parameters =>
     "gid": "nagios",
     "groups": [
       "puppet"
     ],
     "home": "/var/lib/nagios",
     "shell": "/usr/sbin/nologin",
     "system": true
*******************************************
*** End octocatalog-diff on moma.softwareheritage.org

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Migrated from D5065 (view on Phabricator)