apt: lock osstest's usages of apt-get against each other

apt: lock osstest's usages of apt-get against each other

Currently we rely on all apt-get invocations being in a single
ts-xen-build-prep job which can't run on a shared host.

That is a bit inflexible so instead use our own lock. We wait
indefinitely and rely on osstest's existing command timeout
infrastructure to catch problems.

target_install_packages*() previous estimated the time taken to
install the packages based on the number of packages. This no longer
applies because the install might get stuck behind some other large
install. Use a 3000s (nearly an hour) timeout instead (I expect
failures here to be unusual so erred on the big side)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>