Support HCD

[emerkle826]

Management API assumes that either Cassandra or DSE is the server database, and that the effective start command is cassandra or dse (respectively), and that he start command exists in a bin directory under the location pointed to by system environment variable CASSANDRA_HOME or DSE_HOME, again respectively.

Problem:

Since #484, HCD (Hyper-Converged Database) is replacing DSE7. While #485 rebranded DSE7 to HCD, it did not account for new start command binaries or new system environment variables (hcd and HCD_HOME respectively).

Solution:

Management API server core needs to accommodate the new HCD environment and support new start command binaries and system variables