If the userid is not found in the DB do a case insensitive search

[jvlcek]

https://bugzilla.redhat.com/show_bug.cgi?id=1486041

If the appliance administrator manually creates a user in mixed case it would not be found.

This change will do a case insensitive search for the user. To minimize unnecessary
performance degradation, it will only do the case insensitive search if the user can
not be found as presented.

Steps for Testing/QA [Optional]

1. Configure an appliance for authentication Mode: LDAP(s)
2. Manually configure a new user with a mixed case userid.
3. Attempt to log in as that user but enter the username in all upper case.

[jvlcek]

@gtanzillo Please review. Note I also DRYed the code you suggested I might want to from my last PR :)

[miq-bot]

Checked commit jvlcek@7135b43 with ruby 2.2.6, rubocop 0.47.1, and haml-lint 0.20.0
4 files checked, 1 offense detected

app/models/authenticator/base.rb

• ❗ - Line 203, Col 15 - Rails/DynamicFindBy - Use find_by instead of dynamic find_by_userid.

[jvlcek]

@miq-bot add_labels bug

[gtanzillo]

LGTM 👍