Preferences: Multi delete certificates should be possible

It should be possible to delete several certificates in a single UI action

steps to reproduce:

1. ASSERT: make sure that you have accepted and stored at least 2 certificates
2. EXEC: open Eclipse Preferences > JBoss Tools > OpenShift3 > SSL Certificates
3. EXEC: try to select multiple certificates and hit "Delete"

Result:
You cannot select multiple certificates

Expected:
You should be able to select multiple ones and hit "Delete"