Enable it to specify `coinc.xml` instead of `gid` for online automatic PE followups

Download of coinc.xml in automatic PE followups can fail temporary due to high server load. GWCelery automatically retries download if it fails and avoids that problem. Therefore, it is preferred to let GWCelery download coinc.xml and specify its path in an ini file instead of letting lalinference_pipe download it. This patch enables it.

@john-veitch @vivien Could you please review this patch if you have time? I would like to use this patch from the start of O3 if possible for automatic PE followups.