[FEATURE REQUEST] Allow env var "CHECKSD_OVERRIDE" to carry more than one path for checks

[imriss]

Describe the behavior you saw:
Only one path can be supplied to CHECKSD_OVERRIDE env variable.

Describe the behavior you wished:
Capability to supply multiple paths to CHECKSD_OVERRIDE env variable. Probably, using the ":" separator.

Link to the code:

dd-agent/config.py

Line 221 in aa0dc29

path_override = os.environ.get('CHECKSD_OVERRIDE')

Thanks!