[packager] add option to specify keychain in Mac codesign

When codesigning mac apps in the packager the default keychain is presumed to be the location of the desired keys. That is not always true. The codesign command allows for a -keychain option (much like the jarsigner -keystore command) to specify the acceptable keychain to use.

We should add bundler arguments to specify the location of a keychain, and if one is specified whenever codesign is invoked we should use it.