Add methods for adding VTs to the redis cache.

[jjnicola]

What:
Add methods for adding VTs to the redis cache.

Why:
Currently there isn't a method to add elements under a key in redis with the same value, since the methods are being added as set, which does not allow duplicated entries.

How:

from ospd_openvas import db
from ospd_openvas import nvticache

db_c = db.MainDB()
cache = nvticache.NVTICache(db_c)

nt_list = ['some_file.csv', '', '', '', '', '', '', "cvss_base_vector=AV:N/AC:L/Au:N/C:P/I:N/A:N|last_modification=1600178364|creation_date=1600178364|summary=The remote host is missing an update for package(s) announced advisory.|vuldetect=Checks if a vulnerable package version is present on the target host.|insight=Some insight (CVE-1234)|affected=affected|solution=Please install the updated package(s).|solution_type=VendorFix|qod_type=package", 'CVE-1234', '', 'URL:https://developer.com', '3', '0', 'Some family', 'Some vt name)']

cache.add_vt_to_cache(vt_id='1234', vt=nt_list)

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #337 into master will decrease coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #337      +/-   ##
==========================================
- Coverage   84.33%   84.16%   -0.18%     
==========================================
  Files           9        9              
  Lines        1449     1471      +22     
==========================================
+ Hits         1222     1238      +16     
- Misses        227      233       +6     

Impacted Files	Coverage Δ
ospd_openvas/db.py	97.76% <ø> (-1.07%)	⬇️
ospd_openvas/nvticache.py	95.90% <ø> (-2.32%)	⬇️

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