[3282] Addes support for full parsing of disable dhcp-ddns entry

D2CLientConfigParser now supports parsing all parameters even if
enable-updates is false.  This allows values like generated-prefix
and qualifying-suffix to be used even if udpates are turned off.
The parser still supports an abbreviated dhcp-ddns entry that has only
enable-updates defined as false.
[3282] Use DDNS configuration values in b10-dhcp4

Replaced hard-coded constants in dhcp4_srv.cc with methods and
logic in D2ClientMgr to implement behavior changes based on
dhcp-ddns configuration values.
[3282] Removed use of std::string::back

Didn't notice but back() is new and not supported on older
versions of C++.
[3282] Removed use of std::string::back from header

Updated template method to use qualifyName as it should
have been. This also serves to remove use of std::string::back.