releasetools: Support using payload_signer.

For A/B OTAs, by default it calls 'openssl pkeyutl' to sign the payload
and metadata with the package private key. If the private key cannot be
accessed directly, a payload signer that knows how to do that should be
supplied via "--payload_signer <signer>".

The signer will be called with "-inkey <path_to_private_key>",
"-in <input_file>" and "-out <output_file>" parameters.

Test: Use a dummy signer, call 'ota_from_target_files.py --payload_signer <signer> <target_files.zip> <ota.zip>' and verify the signatures in the generated package.
Bug: 28701652
Change-Id: I26cfdd3fdba6fc90799221741b75426988e46fd3
(cherry picked from commit dea0f8bf)