oauth2server: access and refresh tokens encrytion

* Uses encryption field for seamless encryption/decryption of the access
  and refresh tokens. (addresses #2127)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
oauthclient: access token encryption

* Uses encryption field for seamless encryption/decryption of the access
  tokens. (closes #2127)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>