Make ldap login optional

[prabhakarpd7284]

Currently, when the user logins he is redirected to ldap login page and the app first tries to verify it from ldap server. The aim here is to make this choice dependent on user whether he wants to use ldap as default login or mysql db based login.

This can be done by introducing a new boolean environment variable (say ldapLogin) which can be configured as true or false.

https://stillat.com/blog/2018/04/14/laravel-5-retrieving-environment-variables-with-env