Add automatic addition of REQUESTS_CA_BUNDLE to the execution
environment if $SNAP_COMMON/etc/ssl/certs/ca-certificates.crt
is detected, allowing the certs from the core snap to be
overridden on a per snap basis.
This supports use of privately signed certificates in snap
based deployments.
Change-Id: Iac789d25b9a5d0e71ba0afdc8462ff0c2344ef7b
Related-Bug: 1724012