icinga-stack: extraEnvVars value for icinga2/icingadb

[loicwinkopp]

Value to add extra environment variables in icinga2 container, useful to fill variables like PGHOST, PGPORT, PGDATABASE, PGUSER, PGPASSWORD, etc.

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[loicwinkopp]

Done!

[lippserd]

@cla-bot check

[martialblog]

@mocdaniel looks alright to me. In fact, might be good to add this to other charts as well?

[loicwinkopp]

@martialblog @mocdaniel I added it for icingadb (seemed more relevant to me than for icinga web) and added tests

[mocdaniel]

LGTM, thanks for your contribution @loicwinkopp