[#2430] MySQL CB now supports class user-context

[#2430] MySQL CB now supports class user-context

src/share/database/scripts/mysql/dhcpdb_create.mysql
src/share/database/scripts/mysql/upgrade_013_to_014.sh.in
    Added user_context column to class tables

src/bin/admin/tests/mysql_tests.sh.in
    Updated upgrade test

src/hooks/dhcp/mysql_cb/mysql_cb_dhcp4.cc
src/hooks/dhcp/mysql_cb/mysql_cb_dhcp6.cc
src/hooks/dhcp/mysql_cb/mysql_query_macros_dhcp.h
    read/write class user_context

src/lib/dhcpsrv/testutils/generic_cb_dhcp4_unittest.cc
src/lib/dhcpsrv/testutils/generic_cb_dhcp6_unittest.cc
    Added user_context value to class tests