feat: introduce the --user command line option

[edubart]

This makes easier to test the cartesi machine as root, useful you are prototyping with virtio network and want to install packages. Also refactors how the default user is chosen, this is now set by tools init script (this is more appropriate since the user existence depends on it).

$ cartesi-machine --user=root id 

         .
        / \
      /    \
\---/---\  /----\
 \       X       \
  \----/  \---/---\
       \    / CARTESI
        \ /   MACHINE
         '

uid=0(root) gid=0(root) groups=0(root)

Halted

$ cartesi-machine id 

         .
        / \
      /    \
\---/---\  /----\
 \       X       \
  \----/  \---/---\
       \    / CARTESI
        \ /   MACHINE
         '

uid=1000(dapp) gid=1000(dapp) groups=1000(dapp)

Depends on

• feat: set default USER to dapp in init script machine-emulator-tools#39