Fix integer to string conversion

Fix integer to string conversion

Nodeb-rnib unit tests were failing due to illegal integer to string
conversion, when unit tests were tried to run with Golang 1.6 version.
There has been made some updates to string conversion function in
newer Golang version, because string conversion passes with Golang
1.12. With this commit change problematic string() conversion function
to strconv.FormatInt(), which works both with the Golang 1.12 and also
newer versions.

Update also ca-certificates, because they are out-dated in the Ubuntu
18 Docker builder base image.

Fix ci/build export directory creation in Docker image, it should be
created under root directory, not under /tmp directory.

Bump sdlgo version to v0.8.0.

Issue-Id: RIC-836

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: I6e0bad9defd953b286d1e645d543ebf68b4475bf