Description of problem:
It is not possible to enable a repository-set with a repo URL missing the "$basearch" variable. Because of this it is not possible to enable official RHEL 8 Beta repos in Satellite 6.5.

Version-Release number of selected component (if applicable):
Sat 6.5 Snap 11

How reproducible:
always

Steps to Reproduce:
1. Have a manifest with a Red Hat Beta entitlemen uploaded to the Satellite 6.5
2. In "Content -> Red Hat Repositories" enable Beta in the dropbox, and find Red Hat Enterprise Linux 8 for x86_64 - BaseOS Beta (RPMs) (rhel-8-for-x86_64-baseos-beta-rpms).
3. Try to enable the x86_64 repo

Actual results:
The operation fails with a message: "basearch cannot be specified for Red Hat Enterprise Linux 8 for x86_64 - BaseOS Beta (RPMs) as that information is not substitutable in /content/beta/rhel8/8/x86_64/baseos/os" 

Expected results:
Repo is enabled.

Additional info:
The same operation using hammer:

===================================================================
hammer> repository-set info --organization-id 1 --id 7568
ID:                   7568
Name:                 Red Hat Enterprise Linux 8 for x86_64 - BaseOS Beta (RPMs)
Type:                 yum
URL:                  /content/beta/rhel8/8/x86_64/baseos/os
GPG Key:              file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-beta,file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
Label:                rhel-8-for-x86_64-baseos-beta-rpms
Enabled Repositories:

hammer> repository-set enable --organization-id 1 --id 7568
Could not enable repository:
  basearch cannot be specified for Red Hat Enterprise Linux 8 for x86_64 - BaseOS Beta (RPMs) as that information is not substitutable in /content/beta/rhel8/8/x86_64/baseos/os 
===================================================================

The same operation in Sat 6.4 works without any problems.

===================================================================
hammer> repository-set info --organization-id 1 --product-id 25 --id 7568                                                                                                                                           
ID:                   7568
Name:                 Red Hat Enterprise Linux 8 for x86_64 - BaseOS Beta (RPMs)
Type:                 yum
URL:                  /content/beta/rhel8/8/x86_64/baseos/os
GPG Key:              file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-beta,file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release
Label:                rhel-8-for-x86_64-baseos-beta-rpms
Enabled Repositories:

hammer> repository-set enable --organization-id 1 --product-id 25 --id 7568
Repository enabled
===================================================================

The URL really doesn't contain the $basearch var, but Sat 6.4 was apparently much less strict about this (or the $basearch wasn't set by default).