Bug 20612: Make OAuth2 use patron's client_id/secret pairs

Bug 20612: Make OAuth2 use patron's client_id/secret pairs

This patch wires the OAuth related code so it leverages on the new
Koha::ApiKey(s) classes and tools introduced by bug 20568 instead of the
hardcoded entries in koha-conf.xml originally proposed by bug 20402.

To test revisit the test plan for bug 20402, and verify that it works.
But create API key pairs instead of writing them down in koha-conf.xml.
Also:
- Run:
  $ prove t/db_dependent/api/v1/oauth.t
=> SUCCESS: Tests pass!
- Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Benjamin Rokseth <benjamin.rokseth@deichman.no>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>