ovsdb-server: Make database name mandatory when specifying db paths.

ovsdb-server: Make database name mandatory when specifying db paths.

Currently, if we have just one database, we can optionally skip the
database name when providing the DB path for certain options (ex:
--remote=db:[db,]table,column). But in case we have multiple databases,
it is mandatory.

With this commit, we make the database name mandatory. This provides
increased flexibility for an upcoming commit that provides the ability
to add and remove databases during run time.

Feature #14595.
Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>