OSMENG-992 - Implement create model activity

in NS Workflow.

An exception is raised if model already exists

Change-Id: I84dd89850b28287dfefb1abc0d158cc72cd4eb34
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>