Eliminate constructors with side-effects

Eliminate constructors with side-effects

Several of the Gson and Jackson handlers take a GsonBuilder, which
they then configure with additional adapters prior to create a Gson
object that they subsequently use.  The code has been modified so
that the constructors no longer take a GsonBuilder, thus eliminating
the side-effects that they had on the builders.  Instead, a
configBuilder() method has been added to the handler classes that
makes it clear tht they modify the GsonBuilder.

This also has the advantage that classes that want a Gson object
configured per a given handler need only call that handler's
configBuilder() method; previously, they had to constructor the
handler and then retrieve its gson object.

Also updated a few tests to specify the HTTP "Accept" header to ensure
that return results are in yaml format.

Change-Id: I2ef98198041ff9f73913d01ee6ee14ecf20ba617
Issue-ID: POLICY-2081
Signed-off-by: Jim Hahn <jrh3@att.com>