This commit adds a new guest capability, `pebuild_facts`, which loads facts
from guest VMs. If Facter is installed, it will be used to generate a full list
of facts. If Facter is not installed, a basic set of info is computed about the
`os` and `architecture`, which should be sufficient to drive a PE installation.

This commit switches fact retrieval from using `facter` to `puppet facts`. This
smooths over any differences between Facter versions and allows us to capture
things like the Puppet version number and agent certname in addition to custom
facts.

The `vagrant pe-build facts` command prints facts for fact info for every
running VM, or an optional list of target running VMs.

The new `pe_agent` provisioner performs a [Simplified Agent install][1]
for PE 2015.x and newer.

[1]: http://docs.puppetlabs.com/pe/2015.2/install_agents.html#using-the-puppet-agent-package-installation-script

This patch adds a `master_vm` setting to the configuration of the `pe_agent`
provisioner. If set, this setting will be interpreted as the name of another
Vagrant machine which hosts the PE master. Setting `master_vm` will enable the
following tasks during agent installation:

  - The `hostname` of the master machine will be used as a default for the
    `master` setting of the `pe_agent` provisioner.

  - The `pe_agent` provisioner will be able to ensure the correct `pe_repos`
    are configured on the master before attempting agent installation.

This patch updates the acceptance suite to provision CentOS PE 2015.x masters
with `pe_bootstrap` and Ubuntu 2015.x agents with `pe_agent`. Acceptance setup
is also expanded to include support for both CentOS and Ubuntu agent platforms.

Because of the expanded platform support, other tests are also updated to
reflect the change in Vagrant box names.