Allow metadata proxy running with nobody user/group

Allow metadata proxy running with nobody user/group

Currently metadata proxy cannot run with nobody user/group as metadata
proxy requires to connect to metadata_proxy_socket when queried.

This change allows to run metadata proxy with nobody user/group by
allowing to choose the metadata_proxy_socket mode with the new option
metadata_proxy_socket_mode (4 choices) in order to adapt socket
permissions to metadata proxy user/group.

This change refactors also where options are defined to enable
metadata_proxy_user/group options in the metadata agent.

In practice:
* if metadata_proxy_user is agent effective user or root, then:
  * metadata proxy is allowed to use rootwrap (unsecure)
  * set metadata_proxy_socket_mode = user (0o644)
* else if metadata_proxy_group is agent effective group, then:
  * metadata proxy is not allowed to use rootwrap (secure)
  * set metadata_proxy_socket_mode = group (0o664)
  * set metadata_proxy_log_watch = false
* else:
  * metadata proxy has lowest permissions (securest) but metadata proxy
    socket can be opened by everyone
  * set metadata_proxy_socket_mode = all (0o666)
  * set metadata_proxy_log_watch = false

An alternative is to set metadata_proxy_socket_mode = deduce, in such
case metadata agent uses previous rules to choose the correct mode.

DocImpact
Closes-Bug: #1427228
Change-Id: I235a0cc4f0cbd55ae4ec1570daf2ebbb6a72441d