Bug 17274: Display the place where the memcached is picked

Bug 17274: Display the place where the memcached is picked

The memcached config can be read from ENV or koha-conf.xml
In order to remove any ambiguity, let's inform the user which config is
used.

Test plan:
1/ Define the memcached config in ENV and $KOHA_CONF
=> Go on about page, you should get a warning
2/ Define the memcached config in ENV and not in $KOHA_CONF
=> Go on about page, you should get a warning
3/ Do not defined the memcached config in ENV or $KOHA_CONF
=> Go on about page, you should get a warning
4/ Define the memcached config in $KOHA_CONF and not in ENV
=> Go on about page, you should not get a warning, this is the expected
config

Followed test plan, works as expected.
Signed-off-by: Marc <veron@veron.ch>
Signed-off-by: Katrin Fischer  <katrin.fischer@bsz-bw.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>