Add support for overriding SSL keystore for recorder

This commit adds support for passing in java system properties
`gatling.recorder.keystore.path` and
`gatling.recorder.keystore.passphrase`.  If provided, these
values will be used to override the default SSL keystore/certificate
configuration used by the recorder proxy.
Merge pull request #1102 from cprice-puppet/feature/1.4.X/support-overriding-recorder-keystore

Add support for overriding SSL keystore for recorder