Help ansible-pull work better in bootstap environment

Add option to specify inventory.  No default is defined since
ansible-playbook already does this and it allows an ansible.cfg in the
git repository to take precedence.
Overall, this should help ansible-pull work with less setup in advance,
which should be helpful in kickstart scenarios.  Much of this was
discussed in issue #2464.
Update ansible-pull documentation
remove errant and unneeded import of ansible.constants
Merge pull request #3094 from sfromm/issue2464-redux

Updates to ansible-pull and ansible-pull docs