gitlab: support multiple instances

[mdonadoni]

Currently, REANA integrates with only one GitLab instance specified in REANA_GITLAB_HOST. Regarding the launcher, REANA supports gitlab.com, gitlab.cern.ch and REANA_GITLAB_HOST.

We should investigate how to support multiple GitLab instances:

1. For launcher, it is only necessary to make FETCHER_ALLOWED_GITLAB_HOSTNAMES configurable with Helm values
2. For the webhook integration, more work is required, as for example we will need to store multiple secrets for each user

As of now, option 1 is sufficient, while option 2 can be tackled later on. However, it can also make sense to work on both of the options at the same time, if that is more convenient for us.