Task: Cleanup references to master and slave in vt/orchestrator

[deepthi]

There are several references in the code under vt/orchestrator. Some of them can probably be deleted outright because we don't have to worry about backwards-compatibility yet.

For example:

SlaveLagQuery                              string   // Synonym to ReplicationLagQuery

Variables, functions and parameters should be renamed.
Every occurrence should be evaluated to determine whether it can be removed. There might be exceptions like the exact text of MySQL error messages and low-level MySQL commands.

[shlomi-noach]

I have long since removed all and any slave reference in orchestrator, with the exception of:

• API calls (anything that supports slave also supports replica
• API responses (any JSON output that has "slave" also has "replica" or other compatible name)
• CLI commands (anything that supports slave also supports replica)

All the above were kept for backwards compatibility, with the advantage that no new user needs to use the word slave.

vorchestrator can break backwards compatibility and we can eliminate slave terms altogether.

I had/have no work done for removing master.

[GuptaManan100]

Closing this issue, since all the required changes have been made