Speed up fig up

I did some profiling of one of our larger fig setups. I noticed a lot of time wasted on an unnecessary `build()`. This branch adds a flag to disable the `build()` that was called during `fig up`. The slowdown wasn't on the `build()` itself, but the conditional `if not client.images()`, because it requires hitting the docker remote again, and filtering through the images.

In almost every use of fig I perform a `fig build` before a `fig up` already, to ensure that `fig up` is actually running the latest code/containers, so this extra check is completely wasted (and is actually slower than the build itself).

This branch also includes a few cleanup and bug fixes. Some from an earlier `fig tag` branch that probably wont get merged, and some new ones I found while working on the tests.  I'll comment on the specifics inline.

### Background

On this host:

```
$ docker images | wc -l 
1335
$ docker ps -a  | wc -l
104
```
I suspect this issue is less noticeable when there are fewer images, but 1335 doesn't seem like a lot. We actually cleanup images that are greater than 30 days old, so there shouldn't be anything terribly old here.

Total time: 30s
Number of fig services: 8 (2 image, 6 build)
This setup basically does 3 things: 
```
fig pull
fig build
fig up
```

### fig 1.0 profile

This is heavily cleaned up, but shows the relevant lines.

```
    399922 function calls (394317 primitive calls) in 30.741 CPU seconds
    Ordered by: cumulative time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000   16.916   16.916 fig/project.py:170(up)
        6    0.000    0.000   12.334    2.056 docker/client.py:552(images)
        1    0.000    0.000   10.536   10.536 fig/project.py:163(build)
        4    0.000    0.000    2.692    0.673 docker/client.py:716(pull)
```
Tracing the `project.up()` call through the stack:

```
Function                         called...                                                         
                                 ncalls  tottime  cumtime
fig/project.py:170(up)  ->       1    0.000    0.000  fig/project.py:86(get_services)
                                 8    0.000   16.916  fig/service.py:187(recreate_containers)

Function                                          called...                                                         
                                                  ncalls  tottime  cumtime                                              
fig/service.py:187(recreate_containers)  ->       8    0.001    0.277  fig/service.py:77(containers)
                                                  8    0.000   14.839  fig/service.py:171(create_container)
                                                  8    0.000    1.798  fig/service.py:248(start_container)

Function                                       called...                                                         
                                               ncalls  tottime  cumtime                                              
fig/service.py:171(create_container)  ->       8    0.000    1.929  fig/container.py:35(create)
                                               8    0.000   12.910  fig/service.py:329(_get_container_create_options)


Function                                                    called...                                                         
                                                            ncalls  tottime  cumtime                                              
fig/service.py:329(_get_container_create_options)  ->       6    0.000   12.334  docker/client.py:552(images)
```

We see that 12 seconds out of 30 seconds are spent on an operation that we already know to be a no-op


### this branch profile

```
    398171 function calls (392402 primitive calls) in 18.330 CPU seconds
    Ordered by: cumulative time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000   10.927   10.927 fig/project.py:163(build)
        1    0.000    0.000    4.843    4.843 fig/project.py:170(up)
        4    0.000    0.000    1.997    0.499 docker/client.py:728(pull)
```

Time is down to 18 seconds, and `fig up` is now 4 seconds instead of 16.

I set this to always False. I believe since this is in `recreate_container()` we know that an image must already exist for it, since we already have a container. If you think this assertion is incorrect, I can change this to use the param as well.