Update pushysim to use new authn + message formats

[jamesc]

Integration work to make pushysim use hmac-based session authn

• Hook into hmac code
• General cleanups in pushy_common/pushysim of unused code
• Add logic to sign requests to client config endpoint with a user credentials (we use rainbowdash by default) This is configurable in app.config
• Stub implementation of client creation so we can use a different erchef client per worker (does not yet work due to erchef integration issues)
• Remove hardcoded keys for client_public/client_private in pushy + pushysim
• Remove old RSA based message authentication function in pushy_utils

[jkeiser]

MWR.

[markan]

Looks good.