Two small fixes for Runtime ctor

1. Fix sleep parameter defaulting to "Noned"

If the --timeout argument is not specified, 'Noned' gets passed to the sleep command when starting the container runtime.

The patch both updates the command line argument default and changes Runtime.__init__() to default to 1 if None is passed.

Fixes: 4046e06c ("run: Option to override default 1 day timeout")

2. Fix non-iterable default for ssh_agent_keys

The ssh_agent_keys parameter of Runtime.init() defaults to None, but is then used unchecked as a iterable object in a for loop. Trap this case with an exception handler.

Fixes: 9528129c ("Add support for ssh-agent")