fuel plugin: Pick updated gnutls... packages.

Package version 12ubuntu2.4 of libgnutls26_2.12.23,
libgnutls-dev_2.12.23, libgnutls-openssl27_2.12.23,
libgnutlsxx27_2.12.23 are all removed from archive.ubuntu.com and
replaced with package version 12ubuntu2.5

Change-Id: Ie8b823e2f8aed0dd8a54fe6249c43b9f442cee5f
Signed-off-by: Billy O'Mahony <billy.o.mahony@intel.com>
Reviewed-by: Jonas Bjurel <jonas.bjurel@ericsson.com>
Reviewed-by: Mark D. Gray <mark.d.gray@intel.com>
Reviewed-by: Fatih Degirmenci <fatih.degirmenci@ericsson.com>