Add more validations to config file parser

[andrey-malets]

We need more thorough validation of configuration file structure, for example

• check that host/group/people names do not have any forbidden characters;
• issue a warning for hosts which have no admins;
• issue a warning if some person is not defined in people section but used as an admin for some host;
• issue an error of two hosts have overlapping set of MAC addesses

and so on.