Fix isValidCert to work with non RSA private keys
currently the check expects only RSA private keys but newer menshen could serve other keys as well
this updates the check to only look for 'PRIVATE KEY' in the preamble instead of RSA PRIVATE KEY
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currently the check expects only RSA private keys but newer menshen could serve other keys as well
this updates the check to only look for 'PRIVATE KEY' in the preamble instead of RSA PRIVATE KEY