Adapt simple health check to use 3-part image

Adapt simple health check to use 3-part image

This fix adds the possibility to pass custom imformation about the
image via image_metadata variable. In SimpleHealthcheck it checks
for this info and creates a 3-part image if the kernel_url and
ramdisk_url are set. The fix also allows the default url to be
overriden as well as passing extra-properties. All the metadata info
comes from the app.

The part in functest where the custom config is set for creating
a 3-part image is https://gerrit.opnfv.org/gerrit/#/c/29851/

JIRA: ARMBAND-222

Change-Id: I11435d8bab4cf097b4079b6984741869f83aee2b
Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>