plainbox:device: add device module

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:commands:device: add device command

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:box: enable 'plainbox device' command

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:docs: add manual page for 'plainbox device'

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge by tarmac [r=kissiel][bug=][author=zkrynicki]"