Unify certificate loading in EDGAR

[mbfm]

For historic reasons, EDGAR loads certificates for the CarlClient directly, but for NetBird from the OS certificate store. Recently, the OS certificate also started being used for WebDAV, whereas Keycloak again uses the fixed certificate.

This should be changed to only use the certificates from a fixed certificate path.

Related: #249

[reimarstier]

In case of a custom certificate authority it would be best to load it from a common location and store the path to this file in the environment variable SSL_CERT_FILE. This variable should be specified in both, the EDGAR and NetBird service files. See #249