added acquisition to os.environ to enable shortcuts

versions of configman previous to 1.2.4 treated all incoming key/value value_sources as if they had acquisition.  I felt this lead to ambiguity and surprises as common names like 'password' matched unexpected in places.  For example:
* defined options:
 * a.b.password
 * a.c.password
* incoming value_source:
 * "password": "foopass"

configman would assign "foopass" to both a.b.password and a.c.password.

It turns out that crontabber and several features of Socorro elasticsearch tests relied on this feature that I removed.

This PR puts the feature back, but only for environment variables.  If a programmer wants the behavior for other key/value mappings, I suggest explicitly stating so by wrapping the mapping in the class ```DotDictWithAcquisition```.  This fixes the crontabber project.   However, for the Socorro ES tests, I've made sure that they fully qualify their option names so there can be no ambiguity.