Allow X.Y.Z-RC/EC with OSC_CHANNEL candidate in create-prowjob

[tbuskey]

If OSC_CHANNEL=candidate, allow OSC_VERSION to have X.Y.Z-rc/ec-A for testing unreleased OCP versions.

Update the README.md to explain it.
The OCP_VERSION notation was always allowed in the prowjob, but create-prowjob would not generate it

[tbuskey]

/assign @vvoronko
/assign @ldoktor
/assign @wainersm

[tbuskey]

/pj-rehearse ack

[openshift-ci-robot]

@tbuskey: now processing your pj-rehearse request. Please allow up to 10 minutes for jobs to trigger or cancel.

[openshift-ci-robot]

[REHEARSALNOTIFIER]
@tbuskey: no rehearsable tests are affected by this change

Interacting with pj-rehearse

Comment: /pj-rehearse to run up to 5 rehearsals
Comment: /pj-rehearse skip to opt-out of rehearsals
Comment: /pj-rehearse {test-name}, with each test separated by a space, to run one or more specific rehearsals
Comment: /pj-rehearse more to run up to 10 rehearsals
Comment: /pj-rehearse max to run up to 25 rehearsals
Comment: /pj-rehearse auto-ack to run up to 5 rehearsals, and add the rehearsals-ack label on success
Comment: /pj-rehearse list to get an up-to-date list of affected jobs
Comment: /pj-rehearse abort to abort all active rehearsals
Comment: /pj-rehearse network-access-allowed to allow rehearsals of tests that have the restrict_network_access field set to false. This must be executed by an openshift org member who is not the PR author

Once you are satisfied with the results of the rehearsals, comment: /pj-rehearse ack to unblock merge. When the rehearsals-ack label is present on your PR, merge will no longer be blocked by rehearsals.
If you would like the rehearsals-ack label removed, comment: /pj-rehearse reject to re-block merging.

[vvoronko]

/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tbuskey, vvoronko

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[openshift-ci]

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