SIMPLE-6073 broke the ClPyats API contract

[xorrkaz]

With the fix for SIMPLE-6073, CLPyats no longer raises PyatsNotInstalled. The docstring still says that it does, but this will not be raised until sync_testbed() is called. Yet that method doesn't indicate it will raise PyatsNotInstalled.

What might be a simple fix for this is to call _check_pyats_installed() in __init__(). Then the contract is restored and backward compatibility is preserved.