Add check for permissions and better error message

[ondrej-fabry]

Problem

VPP Agent and VPP expect sufficient privileges for some of the features which are usually not enabled by default in the containers and users (especially newcomers) often forget to add them or perhaps don't even realize they are needed.

Because of insufficient privileges, there are different kinds of errors that can show up in the log. Most common ones are:

• VPP cannot allocate huge pages:
  buffer: vlib_physmem_shared_map_create: pmalloc_map_pages: Unable to lock pages: Cannot allocate memory

• VPP Agent is unable to switch network namespace:
  Failed to switch to Linux network namespace (<nil>): operation not permitted

• VPP cannot create TAP interface:
  VPPApiError: System call error #2 (-12)

Solution

VPP Agent should check for required permissions before running operations that require them. If the required permissions are not found then more informative error message should be shown to the user. The error message could include info how to add required permissions.