Fix/issue 41

[ilazakis]

What

1. Fixes issue 'trainingJobName' failed to satisfy constraint #41
2. Corrects a couple of docs typos and adds missing doc entries for docker_tag option.

[ilazakis]

The docker_tag work earlier this year sets the latest docker tag by default unless otherwise specified by the user i.e. there is no need to hardcode it here. This was actually causing image names of the image-name:some-tag:latest form to be used, that Docker and SageMaker declined.

[pm3310]

Hey @ilazakis, it LGTM :-D Great work. You just need to fix some linting issues

$ make lint
flake8
./sagify/commands/__init__.py:8:-153: W605 invalid escape sequence '\_'
./sagify/commands/__init__.py:8:-148: W605 invalid escape sequence '\ '
./sagify/commands/__init__.py:8:-74: W605 invalid escape sequence '\_'
./sagify/commands/__init__.py:8:-68: W605 invalid escape sequence '\_'
./sagify/commands/__init__.py:8:-56: W605 invalid escape sequence '\_'

flake8 doesn't like the ascii logo in the commands/__init__.py but you can ignore it.

Check this out how to ignore this linting issue http://flake8.pycqa.org/en/3.1.1/user/ignoring-errors.html#in-line-ignoring-errors

[ilazakis]

@pm3310 Updated to exclude the ASCII logo file plus a number or usual suspects ala .git etc