secret: Add secret.conf configuration file and parse it

secret: Add secret.conf configuration file and parse it

A new configuration file called secret.conf is introduced to
let the user configure the path to the secrets encryption key.
This key will be used to encrypt/decrypt the secrets in libvirt.

By default the path is set to the runtime directory
/run/libvirt/secrets, and it is commented in the config file.
After parsing the file, the virtsecretd driver checks if an
encryption key is present in the path and is valid.

If no encryption key is present in the path, then
the service will by default use the encryption key stored in the
CREDENTIALS_DIRECTORY.

Add logic to parse the encryption key file and store the key.
It also checks for the encrypt_data attribute in the config file.
The encryption and decryption logic will be added in the subsequent patches.

Signed-off-by: Arun Menon <armenon@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>