Fix datastore-engine help text

[bryanhuhta]

The datastore-engine flag doesn't include MySQL as an option, even though it is legal to select. This PR adds it to the example list.

Also, instead of hard-coding all the possible datastores, I sorted the keys in BuilderForEngine and displayed them. This way, any new datastore will automatically get added to the options list and there won't be any accidental drift.

Before:

--datastore-engine string    type of datastore to initialize ("memory", "postgres", "cockroachdb", "spanner") (default "memory")

After:

--datastore-engine string    type of datastore to initialize ("cockroachdb", "memory", "mysql", "postgres", "spanner") (default "memory")

[josephschorr]

LGTM