environs/instances: expose getMatchingInstanceTypes

The azure provider has some custom instance-type
matching code which can be made to use this function
if it's exposed.
Merge pull request #148 from axw/environs-instances-matchinginstancetypes

environs/instances: expose getMatchingInstanceTypes

The azure provider has some custom instance-type
matching code which can be made to use this function
if it's exposed.

Also, move instance-type constraint matching into `MatchingInstanceTypes` and friends, simplifying `FindInstanceSpec`.