db-ctl-base: Make common database command code into library.

db-ctl-base: Make common database command code into library.

This commit extracts common database command (e.g. ovs-vsctl, vtep-ctl)
code into a new library module, db-ctl-base.  Specifically, the module
unifies the command syntax and common database-operating commands like
(get, list, find, set ...), and provides apis which allow user to create
more specific commands.

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>