vttestserver: Allow override of mysql_server_version

[deepthi]

Feature Description

By default vtgate presents as 5.7.9-Vitess. Frameworks that are used to connect to vitess will sometimes modify the queries they send based on the MySQL version. Vtgate has a flag to allow this to be changed if desired (based on underlying MySQL version).
Right now vttestserver does not allow you to override this version so it always pretends to be 5.7.9-Vitess regardless of the embedded MySQL version.

Use Case(s)

Using vttestserver to test Vitess against MySQL 8.0