packaging: opensuse 15 fix docker and build gnutls

Correct the dockerfile for opensuse 15 replacing
 zypper --non-interactive  --gpg-auto-import-key refresh
by
 zypper --non-interactive  --gpg-auto-import-keys refresh
keyS must be in plural, otherwise the command fails

Update the spec files for opensuse 15 leap by compiling GnuTLS
    make -j4 V=1 .ffmpeg .gnutls

because when using the distro's GnuTLS 3.6.7  connections fail with
[TLS] handshake failed: An error has been detected in the library and
cannot continue operations.

Change-Id: I40a726e429b9f68447199f336ebe4a68ddfccf30