box: add hostname to box.info

Hostname is a useful piece of information in state reports. So it was
decided to add it to box.info.

Hostname is obtained on requested and is not cached.

Closes #8605

@TarantoolBot document
Title: Add hostname to box.info

This patch adds hostname to box.info, it can be useful e.g. to supplement
various instance state reports. It is not cached and is requested on
each call.