Migrate request invocations (openflowplugin)

Migrate request invocations (openflowplugin)

The '* Request' and 'To Json' RequestsLibrary keywords have been
deprecated. Replace '* Request' keywords with '* On Session'.

In the case of 'To Json', replace it with the 'Json Parse from String'
in Utils.robot when the pretty_print is false. When pretty_print is
required, use the Evaulate keyword with the json module to generate
properly formatted JSON string.

JIRA: INTTEST-141
Change-Id: I5a5842bf68870679196824661219be7e62c52d32
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>
Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>