searchidx: mount /srv/mediawiki as a bind from /a

Given the root partition on searchidx is too small to accomodate the
mediawiki checkout, use a bind mountpoint for it

Change-Id: Ib636449eb5baf2c833f1636e2ebc79036650fa4f
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>