MTV-1433: Creating source and destination providers, separated by provider

[RichardHoch]

MTV 2.8

Partially resolves https://issues.redhat.com/browse/MTV-1433

Previews:

• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating-vms-ocp-web-console_mtv [new intro to section]
• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating-vms-mtv-ui_mtv [slightly changed intro]
• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating-vmware-ui [VMware section, all titles my abbreviations]
• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating_virtual_machines_from_rhv [RHV section]
• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating_virtual_machines_from_ostack [OpenStack]
• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating_virtual_machines_from_ova [OVA]
• https://file.corp.redhat.com/rhoch/WIP_section_4/html-single/#migrating_virtual_machines_from_cnv [CNV]

In this PR, I have grouped the procedures for creating source and a destination provider per vendor. You can ignore creating-migration-plans-2.8.adoc. I work with it in #585 .

[RichardHoch]

@sgratch Please review this PR -- just the sections that have previews. You can ignore the changes to the file creating-migration-plans-2.8.adoc. I work with it in #585 . @mnecas --if you have time, please also review.

[RichardHoch]

@mnecas I made the changes you suggested -- the discussion of warm/cold migration is in #585.

Please review this PR again and also #585. Thanks.