Use md-sal datastore for netconf-testtool

Use md-sal datastore for netconf-testtool

There are a few test cases that require netconf-testtool to use
the md-sal datastore. Since the issue with mdsal-netconf-connector
(NETCONF-879) which prevented successful mounting of netconf-testtool
has been addressed, restore the netconf-testtool configuration to use
the md-sal datastore.

This effectively reverts the following change:
Ifd378b8cacf82d71aa81d83e01305d0ce853c597

Change-Id: Idbb2f548bb80c7a66ac3065c27eaa6b72d5b6f79
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>