Use Pupper::Util::Feature to confine ruby-openscap presence

This is much more reliable than just rescuing from the LoadError.
The mechanism of confinement of Puppet feature allows for provider
readiness check.

Addressing:
Warning: Ruby bindings for OpenSCAP could not be loaded.
Info: Caching catalog for fedora20.local.lan
Info: Applying configuration version '1405435392'
Notice: /Stage[main]/Openscap::Package/Package[rubygem-openscap]/ensure: created
Error: Could not set 'present' on ensure: uninitialized constant OpenSCAP at 31:/etc/puppet/environments/production/modules/openscap/manifests/xccdf/eval.pp
Error: Could not set 'present' on ensure: uninitialized constant OpenSCAP at 31:/etc/puppet/environments/production/modules/openscap/manifests/xccdf/eval.pp
Wrapped exception:
uninitialized constant OpenSCAP
Error: /Stage[main]/Openscap::Xccdf::Eval/Xccdf_scan[weekly-ssg-audit]/ensure: change from absent to present failed: Could not set 'present' on ensure: uninitialized constant OpenSCAP at 31:/etc/puppet/environments/production/modules/openscap/manifests/xccdf/eval.pp