Adds support for OSMMON_DATABASE_COMMONKEY to decrypt vim passwords

When vim_accounts msgs arrive in the Kafka bus, vim password is decrypted
and stored in the SQL database. The ideal scenario would be to store it
encrypted in the SQL DB and decrypt it on demand, but that would require
to store the schema_version, instantiate a DbMongo client everywhere it is
needed and a few other modifications that would severely pollute the codebase.
There needs to be a modification in the future on the osm_common side to make
this more doable.

Signed-off-by: Benjamin Diaz <bdiaz@whitestack.com>