set default grafana_url if GRAFANA_API_URL set

[juriku]

Issue Oncall backend calling grafana api when grafana is self hosted and under certain path

fixes errors like:

source=engine:app google_trace_id=none logger=apps.grafana_plugin.helpers.client Error connecting to api instance 404 Client Error: Not Found for url: https://test.com/api/access-control/users/permissions/search?actionPrefix=grafana-oncall-app

while GRAFANA_API_URL=https://test.com/grafana

relevant for when external self-hosted grafana has same URL, but under different path, like /grafana

Also fixes issue when self.api_url has path, but no '/' in the end

if urlparse(self.api_url).path and not self.api_url.endswith('/'):
            self.api_url += '/'

because in case of urljoin(self.api_url, endpoint)
Example:
https://test.com/path, api/v1 -> will become https://test.com/api/v1
https://test.com/path/, api/v1 -> will become https://test.com/path/api/v1

[github-actions]

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[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 30 days if no further activity occurs. Please feel free to give a status update now, ping for review, or re-open when it's ready. Thank you for your contributions!