Fixes #6321/BZ1111695: check permission before displaying bastion page.

Ensure the current user has the correct permission prior to displaying
the Bastion page.  Before this commit the page would display with 403s
for any requests the user did not have access to.

http://projects.theforeman.org/issues/6321
https://bugzilla.redhat.com/show_bug.cgi?id=1111695