Fix race condition with systemd's apt initialization

When LXC container start systemd runs apt.systemd.daily,
which locks dpkg database.

This change implements a 'wait for lock release', so that
install hook doesn't fail when deploying inside LXC.

Change-Id: I71a38ec8ea208f6c9ef1ae3dabda63bd26fa20fa
Closes-Bug: 1656651

hooks/install

@@ -4,9 +4,14 @@
 
 declare -a DEPS=('apt' 'netaddr' 'netifaces' 'pip' 'yaml')
 
+wait_for_dpkg_unlock() {
+    while $(fuser -s /var/lib/dpkg/lock); do sleep .5;done
+}
+
 check_and_install() {
     pkg="${1}-${2}"
     if ! dpkg -s ${pkg} 2>&1 > /dev/null; then
+        wait_for_dpkg_unlock
         apt-get -y install ${pkg}
     fi
 }