keepalived: create internal VRRP session only if needed

Instead of defining an empty hash by default, let's use false to disable
the feature like it was documented in puppet-doc manifest.
Also add a new unit test to validate the keepalived_internal_ipvs VRRP
session is disabled by default.

Also, validate the internal & public VIP are not the same and create a
unit test for this case.
Merge pull request #690 from enovance/disable-internal-vrrp

keepalived: disable keepalived_internal_ipvs by default