Do not allow registration of play cluster

[anjalshireesh]

Description

Since play is a public demo cluster, it should not be allowed to register with subnet.

How to test this PR?

Try to register the play cluster mc license register play --api-key <api-key>

This should fail with error play is a public demo cluster; cannot be registered

Registration of any other cluster should not show this error and should work as expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[shtripat]

LGTM. Verified