Make email sending optional.

[blueandgold]

Based on user feedback, we want to make the email to be optional, especially when running initially for evaluation purposes.

The only other place where the email comes up is in the DM template. Will we assume that email notifications will be required there? If no, i can test it to make sure the optional case is handled correctly.
https://github.com/GoogleCloudPlatform/forseti-security/blob/master/deployment-templates/py/forseti-instance.py

Fixes #146

[carise]

re: your question about DM -- is it possible to test whether the email address properties are present in the context.properties and conditionally add them as commandline flags in the startup script?

[blueandgold]

I was concerned about that, too. Let me verify how it behaves.

[blueandgold]

PTAL, there's a bit more change to make sure we are handling the case when someone does not want to send email in the VM environment. In this case, I am assuming the email properties are not specified. In which case, a string 'None' will need to be handled since None will be formatted to 'None'.

Let me know if there's a better way this can be handled.

[carise]

I'm ok with the current changes, but maybe what you could also do for the DM is create a separate string for the email flags (instead of --sendgrid-api-key {} --email-sender {} etc. just have a {} and string replace the value with either a blank or the full list of flags/values), perhaps?

[blueandgold]

Okay, I shuffled the strings around a bit in the DM template. How does it look now?

[carise]

💯

[blueandgold]

Adding cla: yes label, as the PR author is a googler at the time of this PR was created.