Correct `find_migrate_repo` usage

The `find_migrate_repo` function was never used with an argument although the
function interface presupposes an optional argument named
`package`.

$ grep -r "find_migrate_repo" keystone

    keystone/common/sql/migration.py: repo_path = find_migrate_repo()
    keystone/common/sql/migration.py: repo_path = find_migrate_repo()
    keystone/common/sql/migration.py: repo_path = find_migrate_repo()

The wrong function usage makes the function redundant (only one
string is needed there, all others are never executed) and forces redundant
code lines in other modules.
Database migration functions must take an extension module object and handle
a migration repository path with `find_migrate_repo` function.

::db_sync: [version, [package]]
::db_version: [package]
::db_version_control: [version, [package]]

`find_migrate_repo` raises an error when the migrate repository path doesn't
exist. The error should be handled to prevent further unexpected behaviour.

Closes-Bug: #1273336
Change-Id: Id62a1859a7e72139d25dfe91d29805660b4b7394
Merge "Correct `find_migrate_repo` usage"