GitLab

 - clarified JSON usage in admin guide (True/False not allowed)
 - fixed incorrect reference to hosts4-storage
 - clarified that empty mac-sources entry is not allowed
 - added extra text about syntactic contexts
 - corrected several typos

This change includes the code implemented by the pull request
submitter as well as Marcin's fixes/changes on top of it, but
without a review.

Move Kea logo to left of title in the header.  Also fix an incorrect
cross-reference link in the DHCPv4 chapter.

 - Dhcp4Srv::vendorClassSpecificProcessing removed
 - User's Guide updated
 - disabled obsolete test (will need to be rewritten to take advantage
   of new classification code)
 - classes definitions now use strings for server-name and filename
 - unused configuration removed from unit-tests
 - textFixedFields is now documented
 - Unit-tests now have short descriptions

 - added vendor options example and made it out-of-pool in
   kea6/reservations.json
 - fixed couple indents in dhcp{4,6}-srv.xml
 - added enclosing subnet4 where it was missing in examples
 - renamed section dedicated to option reservations
 - default value of host-reservation-identifiers presented
   differently

Change the connection timeout parameter from an "int" to an "unsigned
int". Update the checks to allow for lexical_cast not throwing an
exception when converting a string representing a negative number
to an unsigned int.

In addition, the default has been changed to five seconds.