Changes

Create Pcu abstract class and make OsmoPcu inherit from it
This base class will be used to describe the required accessors for all 
PCU objects.
It is introduced in this commit and will be further used in the future 
when adding a Dummy PCU object which will be used by NanoBts object.
Change-Id: Ia3fd4551d1f2932362f99f7d44d65f8ae4fd1979

bts_osmo.py: ready_for_pcu is of public access from tests, move it
accordingly
Change-Id: Ie8978854404897ae5de7e3100d55d86c7a5c1df1

Create Bts abstract class and make OsmoBts inherit from it
This base class will be used to describe the required accessors for all 
BTS objects, be it an osmocom BTS or not.
It is introduced in this commit and will be further used in the future 
when adding a NanoBts object.
Change-Id: Ic13133e61abda73a8b507c1a1bd7b98c677460f9

pcu.py: Introduce PcuDummy class
It will be used in later commits by BTS without proper PCU control such 
as NanoBts.
Change-Id: I37ba38f7d81134e5f0ca28fa684fdb09c753bb04

Introduce PowerSupply interface and PowerSupplySispm
File powersupply.py defines the interface to be used by child classes 
implementing it. It also provides helpers to allocate a child class 
based on configuration provided ('type' field).
File powersupply_sispm.py is an implementation using pysispm [1], as
it's the one used to control the programmable power socket we have right
now.
This kind of class will be used in later commits by Nanobts class, as we 
want to poweroff the Nanobts completelly when not in use.
Using it requires the following extra dependencies:
$ apt-get install python3-usb
$ pip3 install pysispm
Related: OS#3040
[1] https://github.com/xypron/pysispm
Change-Id: I981c260eca1a61657147e6d83b4226618088223c

Introduce ip.access nanobts support
Change-Id: Ibaea025b3a503dfe897d36701234445de6d49f82

default-suites.conf: Add nanobts related tests
Change-Id: I054db78e6a71514133e4e3c3158342b74af44201

nanobts: Bind to specific addr instead of specific iface
This way root access is not required.
Change-Id: I02f6fc9e1cb1ea1e1d950eafe22c231a18100f98