Imposed string length limit in sqlalchemy.py

When running gnocchi-dbsync with mysql as the database backend
(v5.5.37), this error comes up:

    "CompileError: (in table 'resource', column 'user_id'):
    VARCHAR requires a length on dialect mysql"

The problem is resolved by setting a limit on the length of columns
of string type created in the database schema.

Change-Id: I8ac472d65ceb502040bee7941618f61d363dd07f