os_info crate

[Algorab]

Using the os_info create prevents to build docker images from scratch.
As far as I can see it is only for building a user_agent String. This can
be done in an other way, or make this via a feature configurable.

[HJLebbink]

@Algorab I could swap it for the sys_info crate, but it might still stop you from building an image from scratch. Although sys_info is lighter than os_info, it might still need some things to be installed.