Update developer-guide for API conformance test

This patch adds the procedure of the manual API conformance test
according to ETSI NFV-TST by using Xtesting.

Issue-ID: SMO-77
Signed-off-by: Koichi Edagawa <edagawa.kc@nec.com>
Change-Id: Ie160553df4c6201f37cd5d58b431b1fe77f1f4fc
1 file changed