all: change bootstrap model name to "admin"

[axw]

When bootstrapping, we now call the initial
model "admin". Some more commands have been
updated to stop using configstore, and more
legacy code has been removed from modelcmd
as a result.

JujuConnSuite had to be updated to name its
initial model "admin" too, to support several
commands which now assume that is what the
admin model is called. As a result, many tests
had to be updated to stop referring to the
initial model as "dummymodel".

(Review request: http://reviews.vapour.ws/r/3966/)

[axw]

Retargeting to feature branch.