Stop creating a new db IMPL for every request.
Stop creating a new db IMPL for every request.

IMPL should be created once, at module load, not once for every
get_instance() call.

Change-Id: Ib2c7888cf9fe074891984e67af83526c994d2e2e

rdo-management/tuskar,rdo-management/tuskar,rdo-management/tuskar,tuskar/tuskar