In python SSTP CLI handler we check certificate existence, but handler does not check this directory or file.
I propose to change `os.path.exists()` to `os.path.isfile()`
https://github.com/vyos/vyos-1x/blob/current/src/conf_mode/vpn_sstp.py#L326-L336
```
if not os.path.isfile(sstp['ssl_ca']):
file = sstp['ssl_ca']
raise ConfigError(f'SSL CA certificate file "{file}" does not exist')
if not os.path.isfile(sstp['ssl_cert']):
file = sstp['ssl_cert']
raise ConfigError(f'SSL public key file "{file}" does not exist')
if not os.path.isfile(sstp['ssl_key']):
file = sstp['ssl_key']
raise ConfigError(f'SSL private key file "{file}" does not exist')
```