Fix SQL errors in SCAP and CERT update (20.08)

[timopollmeier]

What:

• The update of the CVSS score for CERT advisories is only run when the SCAP schema also exists
• Schema prefixes for tables are added to SCAP and CERT alert checks

Why:
This fixes two SQL errors that could when updating the SCAP and CERT data:

• If the CERT update finishes without the SCAP schema existing, e.g. during the first update, the error would be the scap.cves table not existing.
• If the SCAP and CERT alert checks are run after the update, the search_path may only include the default schema, so tables like cves are not found without a prefix (scap.).

How:
Tested by stopping gvmd, dropping the schemas cert and scap, then restarting gvmd to make it rebuild both schemas.

Checklist:

• Tests
• CHANGELOG Entry

[cfi-gb]

I guess this also replaces (at least parts or) #1295 ? Maybe worth to have a look at the PR.

[timopollmeier]

I guess this also replaces (at least parts or) #1295 ? Maybe worth to have a look at the PR.

Yes, there is some overlap but #1295 also changes some other table selections in the SQL.