Description of problem:
Registering RHEL-8 machine with Satellite-6.5 using bootstrap script with --rex and --rex-user parameter fails with error 'TypeError: 'in <string>' requires string as left operand, not bytes'

Version-Release number of selected component (if applicable):
Satellite-6.5

How reproducible:
Always with RHEL-8 client

Steps to Reproduce:
Download the boostrap script for Satellite-6.5 on RHEL-8 client machine and execute it like below.

# /usr/libexec/platform-python bootstrap.py -l admin -s 'satellite.example.com' -o 'redhat' -L 'default_location' -g bootstrap -a Test_AK --rex --rex-user=root --unmanage --force  --skip puppet  --skip-foreman

Actual results:
The client gets successfully registered but fails at below step while installing Remote Execution SSH key for user

[SUCCESS], [2019-10-21 22:44:22], [/usr/bin/yum -y remove rhn-setup rhn-client-tools yum-rhn-plugin rhnsd rhn-check rhnlib spacewalk-abrt spacewalk-oscap osad 'rh-*-rhui-client' 'candlepin-cert-consumer-*'], completed successfully.
[NOTIFICATION], [2019-10-21 22:44:22], [Fetching Remote Execution SSH key from https://syssat65.gsslab.pnq2.redhat.com:9090/ssh/pubkey]
[NOTIFICATION], [2019-10-21 22:44:23], [Installing Remote Execution SSH key for user root]
Traceback (most recent call last):
  File "bootstrap.py", line 1563, in <module>
    install_ssh_key_from_url(remote_exec_url)
  File "bootstrap.py", line 674, in install_ssh_key_from_url
    install_ssh_key_from_string(foreman_ssh_key)
  File "bootstrap.py", line 708, in install_ssh_key_from_string
    if foreman_ssh_key in open(options.remote_exec_authpath, 'r').read():
TypeError: 'in <string>' requires string as left operand, not bytes