Add support for downloading custom packages via HTTP

Adds support for downloading custom packages for Stanchion over HTTP via the `stanchion_custom_package` attribute.

I tested these changes by creating symlinks to the `ansible-stanchion` working directory from the `examples/roles` directory of the `ansible-riakcs` role:

```bash
$ pwd
/Users/hector/Projects/ansible-riakcs/examples
$ ls -l roles
total 24
drwxr-xr-x  16 hector  staff  544 May 23 22:46 basho.riak
drwxr-xr-x  12 hector  staff  408 May 23 22:46 basho.riak-common
lrwxr-xr-x   1 hector  staff    6 May 26 11:13 basho.riakcs -> ../../
lrwxr-xr-x   1 hector  staff   32 May 27 14:19 basho.riakcs-packages -> ../../../ansible-riakcs-packages
lrwxr-xr-x   1 hector  staff   26 May 27 14:33 basho.stanchion -> ../../../ansible-stanchion
```

And then running:

```bash
$ vagrant destroy -f && ANSIBLE_RIAK_CS_OS="UBUNTU" vagrant up
$ vagrant destroy -f && ANSIBLE_RIAK_CS_OS="CENTOS" vagrant up
```