fix(clapi): use downtime_id to show and cancel rtdowntimes (19.x)

[cgagnaire]

Description

The CLAPI commands "-o RTDOWNTIME -a show -v "HOST"" and "-o RTDOWNTIME -a show -v "SVC"" use the internal_id for the downtimes, instead of the downtime_id which is unique.
Same for "-o RTDOWNTIME -a cancel".

This way, you cannot cancel the exact downtimes you want.

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
• 20.04.x (master)

How this pull request can be tested ?

• Add several downtimes on several hosts and services of several pollers
• Use CLAPI to list real-time downtimes
• Try to delete some of those downtimes using listed IDs
• You will delete the downtimes you want (The external commands in engine will use the internal_id)

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)