Accept custom secret with Nexus credentials to be used instead of default admin

[ricardozanini]

Is your feature request related to a problem? Please describe.
From 0.3.0 we will create the Nexus user and perform some operations in the Nexus server with the REST API. The first interaction needs the default Nexus credentials, which can be viewed as a bad practice. Would be nice to accept a secret with the credentials of a custom admin user for the operator to use instead.

This should be somewhat easy to implement, instead of using the admin/admin123 credentials, we fetch from already defined Secret by the user. The changes will be basically in this package:
https://github.com/m88i/nexus-operator/blob/v0.3.0/pkg/controller/nexus/server/users.go#L72-L104

Describe the solution you'd like
See above.

Describe alternatives you've considered
N/A

Additional context
See #5