[RelEng] Disable validation when adding child to composite repository

Not verifying the existing of the child to add can increase the
robustness of the operation (it guards against false negative existence
check results due to server-side caching issues).
And the added child repositories are expected to always exist anyways,
otherwise it's a programming/configuration error.

Author: HannesWell

JenkinsJobs/Releng/modifyP2CompositeRepository.jenkinsfile

@@ -46,6 +46,7 @@ pipeline {
 						-Dp2.composite.children.add=${params.add} \\
 						-Dp2.composite.children.remove=${params.remove} \\
 						-Dp2.composite.children.limit=${params.sizeLimit ?: 0} \\
+						-Dp2.composite.validate=false \\
 						${params.repositoryName ? ("-Dp2.repository.name='" + params.repositoryName + "'") : ''} \\
 				"""
 					sshagent(['projects-storage.eclipse.org-bot-ssh']) {