Update percona 5.6 key (#2181)

* Update percona signing key as per: https://www.percona.com/blog/2016/10/13/new-signing-key-for-percona-debian-and-ubuntu-packages/

* Update signing keys for percona 5.6

docker/bootstrap/Dockerfile.percona

@@ -1,8 +1,8 @@
 FROM vitess/bootstrap:common
 
 # Install Percona 5.6
-RUN apt-key adv --keyserver ha.pool.sks-keyservers.net \
-        --recv-keys 430BDF5C56E7C94E848EE60C1C4CBDCDCD2EFD2A && \
+RUN apt-key adv --keyserver keys.gnupg.net \
+        --recv-keys 8507EFA5 && \
     add-apt-repository 'deb http://repo.percona.com/apt jessie main' && \
     { \
         echo debconf debconf/frontend select Noninteractive; \

docker/lite/Dockerfile.percona

@@ -2,8 +2,8 @@
 FROM debian:jessie
 
 # Install dependencies
-RUN apt-key adv --keyserver ha.pool.sks-keyservers.net \
-        --recv-keys 430BDF5C56E7C94E848EE60C1C4CBDCDCD2EFD2A && \
+RUN apt-key adv --keyserver keys.gnupg.net \
+        --recv-keys 8507EFA5 && \
     echo 'deb http://repo.percona.com/apt jessie main' > /etc/apt/sources.list.d/mysql.list && \
     { \
         echo debconf debconf/frontend select Noninteractive; \