b2c.hostname: Allow setting the hostname via the kernel commandline

Right now, the hostname in containers can be set by adding -h <hostname> in the podman command line. However, if one has an entire pipeline of containers, it may be better to set it once at the b2c level.

I thus propose introducing b2c.hostname as a simple way to overwrite the current hostname. Also, I propose changing the default hostname from dut to b2c or boot2container.