Allow cmdlets to use default cert/keys

[codyhosterman]

With the 1.4.x.x release there is a new default cert/key feature. The cmdlets should be enhanced to not require a cert or private key if the default is desired. Cmdlets:

Get-PureOnePublicKey, New-PureOneJwt, New-PureOneConnection

[codyhosterman]

This is complete ins 1.4.2.0 with help examples added.