[sdk-manage] Add support for ssu register

To register the build engine and all sb2 targets that require
registration:
$ sdk-manage --register-all --user <username> --password <password>

To register one, multiple or all sb2 targets:
$ sdk-manage --target --register --user <uname> --password <pass> --all | <tname> [<tname>...]

To register the build engine:
$ sdk-manage --sdk --register --user <name> --password <pass>

In addition to the register changes, adjust listing targets so that
only targets that are accessible from where sdk-manage is called are
listed.

The accessibility check is required because the user can have sb2
targets that are created on filesystems not accessible by the build
engine.
Merge pull request #1 from sailfish-sdk/sdkregister

[sdk-manage] Add support for ssu register