[6.15.z] redhat_repositories search box & button xpath, add wait_for in search

[Satellite-QE]

Cherrypick of PR: #1315

Problem: Search box and search button for Content > Red Hat Repositories page were not found, rather script was selecting left pannel search box. Apart from this, test was not waiting for page to load before searching on Red Hat Repositories page.

Solution: The updated XPaths and added wait_for method before searching any rh repositories.

Note:
Correspondig Robottelo PR SatelliteQE/robottelo#14673

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/ui/test_repository.py -k test_positive_delete_rhel_repo
env:
    ROBOTTELO_server__deploy_arguments__deploy_sat_version: '6.15'
    ROBOTTELO_server__deploy_arguments__deploy_snap_version: '13.0'