Set "remove rule" field to be required in dialog

The dialog can be open from: the "Firewalls"->"Firewall Policies"
->"More" button of a policy->"Remove Rule"

The "remove rule" field should be required, otherwise, user will
hit error when no rule is select. Removed "required=False",
because it is default to True.

Added 'btn-danger' to the button style

Change-Id: Ifa21419bc93818335057bb16ed2683994c6efee7
Closes-Bug: #1252996
Merge "Set "remove rule" field to be required in dialog"