Replace migratedb with initdb for database initialization

[golamhisham]

This PR replaces the migratedb call with initdb in migrate.py to ensure the default behavior of Airflow database initialization is used.

This is my first open-source contribution!

[MazrimT]

initdb is depricated with the following DepricationWarning:

"`db init` is deprecated.  Use `db migrate` instead to migrate the db and/or "
"airflow connections create-default-connections to create the default connections",

[FrancisLee13]

As mentioned in issue#223, we should only change this for next version release, not the existing versions.

initdb is depricated with the following DepricationWarning

I was thinking of calling initdb in airflow/utils/db.py, which doesn't have deprecation warning. Although I am not sure if that is also on deprecation path as well and just not marked yet. We should decide when we are working on the next airflow version.