Update mazer internals to focus on Collection instead of Repository (renaming, etc)

[alikins]

Most of the mazer internals refer the "the thing that represents an installable unit" as a Repository, since it currently maps to http://localhost:8000/api/v1/repositories/
and previously meant both trad roles and 'multi-content-collections.

Now collections are collections and distinct from a type of Repository (/v2/collections/ etc).

Since in the future referring to the installable units in mazer as 'repositories' will be confusing,
the names used inside the code should be updated to be focused around Collection.

This is mostly a renaming issue, plus a few cases where the behavior is different (rest_api, fetch/galaxy_url, etc).

MAZER VERSION

name = mazer
version = 0.4.0
config_file = /home/adrian/.ansible/mazer.yml
uname = Linux, newswoop, 4.18.16-100.fc27.x86_64, #1 SMP Sun Oct 21 09:33:00 UTC 2018, x86_64
executable_location = /home/adrian/venvs/galaxy-cli-py3-2/bin/mazer
python_version = 3.6.6 (default, Jul 19 2018, 16:29:00) [GCC 7.3.1 20180303 (Red Hat 7.3.1-5)]
python_executable = /home/adrian/venvs/galaxy-cli-py3-2/bin/python