Adjust apache proxy to treat gerrit requests exactly as given

When trying to add "ldap/ops", the REST PUT request ends in
"[...]/ldap%2Fops", so apache directly answers with 404 due
to the encoded slash. However, allowing encoded slashes only
leads to encoding problems stemming from ProxyPass. Gerrit
expects "[...]/ldap%2Fops".

* AllowEncodedSlashes Off
This is what we had per default, and leads to gerrit not seeing the
request.

* AllowEncodedSlashes On
Gerrit gets "[...]/ldap/ops", and as it splits at "/", gerrit tries to
add the non-existing group "ldap".

* AllowEncodedSlashes NoDecode
Gerrit gets "[...]/ldap%252Fops" ... double encoded.

Change-Id: Ia15c1103bef7dfd9aabb45efb8ea0ea37ac924a3
Merge "Adjust apache proxy to treat gerrit requests exactly as given" into production