common: fix retry on raw install python for rhel

common: fix retry on raw install python for rhel

when the following failure is thrown
```
    rhel-8.0.0-beta-1.7-     [===                 ] ---  B/s |   0  B     --:-- ETArhel-8.0.0-beta-1.7-appstream                   0.0  B/s |   0  B     00:00
    rhel-8.0.0-beta-1.7-     [===                 ] ---  B/s |   0  B     --:-- ETArhel-8.0.0-beta-1.7-baseos                      0.0  B/s |   0  B     00:00
    rhel-8.0.0-beta-1.7-     [   ===              ] ---  B/s |   0  B     --:-- ETArhel-8.0.0-beta-1.7-builder                     0.0  B/s |   0  B     00:00
    Failed to synchronize cache for repo 'rhel-8.0.0-beta-1.7-appstream', ignoring this repo.
    Failed to synchronize cache for repo 'rhel-8.0.0-beta-1.7-baseos', ignoring this repo.
    Failed to synchronize cache for repo 'rhel-8.0.0-beta-1.7-builder', ignoring this repo.
    No match for argument: python3
    Error: Unable to find a match
```

dnf returns 0 anyway.

Let's ensure the pattern 'Failed' isn't present in the output.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>