fix(downtimes): apply dt on resources linked to a poller

[lpinsivy]

Description

You can't set a downtime on a Poller using the popin form on :
"Monitoring > Downtimes > Downtimes" and selecting "Pollers", as no selectbox is available

Type of change

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software
• Updating documentation (missing information, typo...)

Target serie

• 2.8.x
• 18.10.x
• 19.04.x
• 19.10.x (master)

How this pull request can be tested ?

Create a downtime and select a poller.
Then check the centengine.log of the selected poller and check that the command has been sent to centengine.

Checklist

Community contributors & Centreon team

• I followed the coding style guidelines provided by Centreon
• I have commented my code, especially new classes, functions or any legacy code modified. (docblock)
• I have commented my code, especially hard-to-understand areas of the PR.
• I have made corresponding changes to the documentation.
• I have rebased my development branch on the base branch (master, maintenance).

Centreon team only

• I have made sure that the unit tests related to the story are successful.
• I have made sure that unit tests cover 80% of the code written for the story.
• I have made sure that acceptance tests related to the story are successful (local and CI)

[kduret]

don't forget to fill the description ;)

[sc979]

@kduret -> I've updated the description. Is it enough ?