Description of problem:
Bootstrap script fails with an error `SSL: TLSV1_ALERT_UNKNOWN_CA] tlsv1 alert unknown ca (_ssl.c:579)` while migrating systems registered with portal(using RHSM) to Red Hat Satellite 6. 

~~~
[RUNNING], [2018-04-07 23:21:20], [/usr/sbin/subscription-manager register --org 'Orion' --name 'm1.sat6.gsslab.pnq.redhat.com' --activationkey 'RHEL7'  --serverurl=https://satellite62.example.com:443/rhsm --baseurl=https://satellite62.example.com/pulp/repos --force] 
[ERROR], [2018-04-07 23:21:21], EXITING: [/usr/sbin/subscription-manager register --org 'Orion' --name 'client1.example.com' --activationkey 'RHEL7'  --serverurl=https://satellite62.example.com:443/rhsm --baseurl=https://satellite62.example.com/pulp/repos --force] failed to execute properly.
Unable to verify server's identity: [SSL: TLSV1_ALERT_UNKNOWN_CA] tlsv1 alert unknown ca (_ssl.c:579)
Unregistering from: satellite62.example.com:443/rhsm
~~~

Version-Release number of selected component (if applicable):
Satellite 6.2

How reproducible:
Always

Steps to Reproduce:
1.Register any RHSM compatible system to the portal.
2.Now, use bootstrap script to migrate it over Satellite 6.

Actual results:
Unable to verify server's identity: [SSL: TLSV1_ALERT_UNKNOWN_CA] tlsv1 alert unknown ca (_ssl.c:579)
Unregistering from: satellite62.example.com:443/rhsm


Expected results:
System should get registered to satellite6 without any issues.

Additional info:
This issue occurs because bootstrap script is unable to unregister system from a previously registered parent. i.e RHSM.
The reason why it is unable to unregister system is, we are changing rhsm.conf entries with 'katello-ca-consumer-latest.noarch.rpm' prior to unregistering the system.