Migration DB_INIT_VERSION in common place

Init version value exists only in migration_helpers. migration_helpers
shouldn't know nothing about migrate repos, primarily because it is
intended for handling of any migrate repo in the project.

Change-Id: I8bceea0cf5eaf8e623efbb39e385b1990eca0a6f
Closes-Bug: #1308593
Merge "Migration DB_INIT_VERSION in common place"