Certificate parameter cleanup

The -CertificateThumbprint and -NewCertificateThumbprint parameters of all commands other than Get-KeyEncryptionCertificate are now deprecated, and thumbprints can be passed directly to the -Certificate and -NewCertificate parameters instead.  In addition, you can pass paths to certificate files on the file system or in the Cert:\ drive, or X509Certificate2 objects that were loaded by any other means.

In a future release, the old Thumbprint parameters will be removed, for a simpler API.