Make the provisioning setup easier to configure, by
allowing the user to specify PROVISIONING_INTERFACE
variable at runtime. Also handle interface names
like br-ex or eth0.1234 that facter mangles.

In lieu of changing foreman-installer code, provide facterized if name to installer.pp
Merge pull request #116 from huit/features/provisioning-interface

Provisioning setup easier to configure via explicitly define provisioning interface