user creation is not atomic

[ndegory]

When creating a user on a platform without the --registration=none option and without a valid sendgrid key, the user gets corrupted, it's not there, but it's not there.

$ amp -k -s xxx user signup --name nicolas
[xxx:50101]
Error: SendGrid API key is empty
# adding a new secret with a valid sendgrid key, and updating the amp_amplifier service with this secret
$ amp -k -s xxx user ls
[xxx:50101]
USERNAME   EMAIL   CREATED ON
su                 25 Oct 17 13:41
$ amp -k -s xxx user signup --name nicolas
[xxx:50101]
Error: Error updating object: user already exists