Add resource to manage roles of existing users

[interone-ms]

This PR resolves #340 in a generic way, by providing a distinct resource to manage roles of an existing user.

This avoids in large installations with users not managed in Terraform the need to import users into Terraform manually so that roles can be managed.

In installations where users are provisioned by LDAP and so cannot be managed by Terraform it enables managing roles in the first place.

Note: Using nexus_security_user_roles and nexus_security_user at the same time will cause drift issues.

(This PR is based on and thus includes the logging changes in #353)

resource "nexus_security_user_roles" "myuser" {
	userid    = "myuser"
	roles     = ["nx-anonymous", "myrole"]
}

[anmoel]

lgtm

[anmoel]

hi @interone-ms,

thanks for the PR. It's looks good but the new acceptance test is failing. please fix it.

thank in advance and kind regard
André