[2.3] MAAS improperly determines the version of some installs

Due to a bug in `get_version_from_apt()`, which attempts to find the version of a "primary" MAAS package, some installations can fail to detect their own version as follows:

>>> from provisioningserver.utils.version import get_version_from_apt
>>> get_version_from_apt('maas-region-api', 'maas-rack-controller')
''
>>> get_version_from_apt('maas-rack-controller')
'2.3.0~alpha3-6250-g58f83f3-0ubuntu1~16.04.1'

The following example was taken from a machine with only the rack controller installed. MAAS will look for the versions of maas-region-api, then maas-rack-controller, in the apt cache.

Unfortunately, apt considers the package to be in the cache whether or not it is actually installed, and returns an empty string as the version, which the MAAS code didn't expect.