Allow memcached to be enabled on the 'make check' command line.

So far, tests could be run with memcached by tweaking the file
subversion/tests/tests.conf before running the tests.
With this commit, a running memcached server can specified directly
on the command line, like so:

  make check MEMCACHED_SERVER=127.0.0.1:11211
 
This is similar to how other test options are enabled/disabled.

Currently, a lot of tests fail if this option is used, mostly due
to checksum mismatch errors like:

W: svn: E200014: Checksum mismatch for '/home/stsp/svn/svn-trunk/subversion/tests/cmdline/svn-test-work/working_copies/basic_tests-19/A/D/gamma':
W:    expected:  412138bd677d64cd1c32fafbffe6245d
W:      actual:  ada10d942b1964d359e048dbacff3460
W: 

I believe the reason for such failures is that the test suite copies an
existing repository for each test without changing the repository UUID.
Memcached cache might return wrong fulltexts if a repository UUID is reused.

* Makefile.in
  (check): Pass $(MEMCACHED_SERVER) to run_tests.py's --memcache-server flag.

* build/run_tests.py
  (usage, main): add --memcached-server option
  (TestHarness): Add memcached_server parameter and pass it on
   to snvtest.main.options.

* subversion/tests/cmdline/svntest/main.py
  (create_repos): For >=1.6, enable use specified memcached in fsfs.conf
   if --memcached-server was provided.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1579376 13f79535-47bb-0310-9956-ffa450edef68