Remove "Generex" dependency

Remove "Generex" dependency

Removed "Generex" and replaced with direct usage of "Automaton"
to generate random strings as "Genegex" seems to be no longer
actively developed project.

In fact the previous logic to generate example strings was broken.
We have fixed it by using RegularExpressionString instead of
JavaPatternString.

Now we have the same examples generated every run which helps us
to write better tests with assertions.

JIRA: NETCONF-980
Change-Id: I52e6d9d1a0f0cf43eb17b99309d9101c32d75a42
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>