Ensure certain perl modules (and git) are installed on exec nodes

Three of these are already installed by dependencies, but I'm told the
policy is that we want to make it explicit when something uses a
package directly. AnomieBOT uses these.

* libdbi-perl - required by libdbd-mysql-perl
* libhttp-message-perl - required by libwww-perl
* libtimedate-perl - required by libhttp-date-perl which is required by
  libwww-perl
* libxml-parser-perl isn't strictly required by anything installed,
  although there's a path involving a Recommends at one step from both
  libxml-libxml-perl and libxml-simple-perl
* git - I expect this is being required somewhere, but I don't know
  where.

Change-Id: I0f0575f4c6aea8d6f27b791ecb24f338188bbfd2