[fix][slack] send notification even if build `not_built`

current setup failed to send webhook if build status not in:
success,failure or unstable. It breaks messaging if build status:
`not_built`
us: https://mirantis.jira.com/browse/PRODX-3254

Change-Id: Ifabbdcbe7851c16e5ce1cc2b3b125add9e0513f9
1 file changed