Revamp ansible roster to use ansible-inventory

[gtmanfred]

What does this PR do?

Ansible now provides an ansible-inventory command which renders the inventory script/file. Because of this, we can now use that command to render the inventory for ourselves, instead of having to maintain code to process the file as well.

Tests written?

Yes

Commits signed with GPG?

Yes

[isbm]

I am really wondering why we would execute that in so heavy-weight fashion spinning off another Python interpreter etc, instead of just reuse the ansible.cli.InventoryCLI for this. As a conceptual example:

try:
    import ansible.cli.inventory as ansible_inventory
except
    ansible_inventory = None

def __virtual__():
    return ansible_inventory and __virtualname__, 'Ansible is not installed on your system'

def ...:

    c = ansible_inventory.InventoryCLI(['', '-i', get_roster_file(__opts__), '--list'])
    c.parse()
    inventory = c.run()

    ...

[gtmanfred]

We still support python 3.4, which ansible does not, so in order to use ansible-inventory on centos 7, right now you have to pip2 install ansible, and then run the binary.

Until we drop python3.4 support i think this is the better setup.

[isbm]

Ah, you're right. CentOS 7 and family on 3.4. OK, in this case we should do the fallback then:

return (ansible_inventory or which('ansible-inventory')) and __virtualname__, '....'

I still don't want to spin-off another Python process on an expensive IBM Mainframe, where every bit of RAM is for gold. 😉 Once we stop 3.4 support, we can just drop this part.

[gtmanfred]

.tox/py36/lib/python3.6/site-packages/ansible/cli/inventory.py:186: in run
    exit(0)
_ _ _ _ _ _ _ _

This is why we cannot use the ansible/cli/inventory.py script, because it calls a system exit. and just displays the results instead of returning them.

[terminalmage]