(PUP-2878) file conflict upgrading from 2.7.26 to 3.6.2

Currently, upgrades from 2.7.26 to 3.6.2 break because of a file
conflict. It seems that as of Puppet 3.3.0, the manpages changed from
being owned by the puppet package, to being owned by the puppet-common
package. This change was initially done to allow upgrades from Debian
packages to ours. However, changing which package owns a given file
makes upgrades difficult. This commit adds in conflicts to allow smooth
upgrades from puppet packages prior to the manpage change to the current
version.