Restore SAML enterprise user auth logic
Restore SAML enterprise user auth logic

Connect #7666 of ansible-tower and follow up original fix tower #455.
The original fix solves the problem of duplicated db keys, but breaks a
rule of enterprise users that 'Enterprise users cannot be
created/authenticated if non-enterprise users with the same name has
already been created in Tower.'. This fix resumes that rule.

Signed-off-by: Aaron Tan <555f4d7ad24fb44b9d01fe2d4954b2d18e7bdbd4@gmail.com>

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