improve ansible start up time for very large inventory
improve ansible start up time for very large inventory

first off, we add an oddly slow basic test of 10k item inventory

Before:
```
Ran 229 tests in 13.214s

OK

real    0m13.403s
user    0m12.106s
sys 0m1.155s
```

After:
```
Ran 230 tests in 21.328s

OK

real    0m21.516s
user    0m20.099s
sys 0m1.275s
```

since that seems like a bit long for the test to add to runtime, lets profile

`python -m cProfile -s time ./bin/ansible all -i test/units/inventory_test_data/huge_range --list-hosts`

Before:
```
         1272607 function calls (1259689 primitive calls) in 8.497 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    10000    4.393    0.000    4.396    0.000 __init__.py:395(_get_host)
    20000    2.695    0.000    2.697    0.000 __init__.py:341(__append_host_to_results)
    40369    0.113    0.000    0.113    0.000 {posix.lstat}
    50006    0.102    0.000    0.153    0.000 __init__.py:1490(combine_vars)
    40008    0.089    0.000    0.202    0.000 __init__.py:1546(_load_vars_from_path)
    20195    0.088    0.000    0.088    0.000 {posix.stat}
    10011    0.087    0.000    0.087    0.000 {posix.getcwd}
```

The top two lines are promising optimization targets

- populate Inventory's host cache more in _get_host, as we are looping
  over all the groups anyways.

- eliminate duplicate check of whether we've already included a host
  in the construction around __append_host_to_results we can infer
  presence of a host in the results list implies the presence of its
  name in the hostnames set, allowing us to only to the less expensive
  of the two checks

After:
```
         1252610 function calls (1239692 primitive calls) in 1.320 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    40369    0.105    0.000    0.105    0.000 {posix.lstat}
    50006    0.094    0.000    0.141    0.000 __init__.py:1490(combine_vars)
    40008    0.081    0.000    0.184    0.000 __init__.py:1546(_load_vars_from_path)
    10011    0.080    0.000    0.080    0.000 {posix.getcwd}
    20195    0.074    0.000    0.074    0.000 {posix.stat}
    10002    0.069    0.000    0.261    0.000 __init__.py:1517(load_vars)
```

thaim/ansible,thaim/ansible