Refactor pretty printing

Refactor pretty printing

PrettyPrintParam is a common operation, introduce FormatParameters
to expose it. This interface will be useful for controlling other
parameters as well.

This really makes ReplyBody a FormattableBody, so we perform this
renaming as well and make FormatParameters a required argument.

This flushes out the fact we are not accepting this parameter for invoke
operations. Correctly propagate these parameters, which forces us to
clean up the RPC/action invocation paths -- further simplifying the
interface and creating proper place where these parameters get applied.

JIRA: NETCONF-773
Change-Id: I3382b27c0b4f7d727d792ead5c1ad364bd69624c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>