Fix optional.txt issue on Open edX dogwood installation

Well, last week dogwood version of Open edX have broken installation due to Google removing an asset we were using during installation, but Open edX team has been fixed this issue which is available in named-release/dogwood.1. If you try new installation and get error like this

TASK: [edx_notes_api | install application requirements] ********************** 
ok: [localhost] => (item=base.txt)
failed: [localhost] => (item=optional.txt) => {"cmd": "/edx/app/edx_notes_api/venvs/    edx_notes_api/bin/pip install --exists-action w -r /edx/app/edx_notes_api/edx_notes_api/    requirements/optional.txt", "failed": true, "item": "optional.txt"}
msg: 
:stderr: Could not open requirements file: [Errno 2] No such file or directory: '/edx/app/    edx_notes_api/edx_notes_api/requirements/optional.txt'
You are using pip version 7.1.2, however version 8.1.0 is available.
You should consider upgrading via the 'pip install --upgrade pip' command. 

It means the option.txt file has been removed on top configuration app but not updated on edx-notes-api repository so the problem is that edx-notes-api is not versioned to dogwood, so we are getting master of edx-notes-apirather than the version of the code that worked with Dogwood.

To fix this issue is simple by using cherry-pick from git to this commit https://github.com/edx/configuration/commit/de9dab63381ae036304bb25c04a65f3a689902eb. Cherry-pick is designed to apply some commit from one branch into another branch. It can be done if you eg. made a mistake and committed a change into wrong branch, but do not want to merge the whole branch. You can just eg. revert the commit and cherry-pick it on another branch.

Go to /var/tmp/configuration and make sure you are working on dogwood.1 branch then run command

git cherry-pick de9dab6

or if you use automatic installation using sandbox.sh download the file first by using wget and add git cherry-pick under git checkout $CONFIG_VER line, the code will look like this.

cd /var/tmp
git clone https://github.com/edx/configuration
cd configuration
git checkout $CONFIG_VER
git cherry-pick de9dab6

Run your installation again, Done.