Read options from .omcrc, allow default account

Account now needs to be specified as an option. As:

  omc -a freerunningtech staging ssh

or

  omc --account=freeruningtech staging ssh

The following paths are searched when looking for configuration, and
are loaded in that order.

~/.omcrc
~/.config/omcrc
.omcrc

Change-Id: Id2dee614ea96e51b42e0fb00e8770d4933fb4822
Specify required options in descriptions

Change-Id: I88a64508956a3f8c99efe249f60f998febe7f935