Cleanup and update to PHP5 class conventions

Should throw exceptions on errors that prevent proper `__cosntruct()` execution.
Should throw exceptions on errors that prevent proper execution of other logic, unless actual error handling
    is implemented internally to such logic.

Also minimized operation overhead by moving initial node list loading to end of successful login, instead of
    on every action possible post-login.  (There were checks to determine if upstream requests were required
    to fulfill this need, but that check itself presents an amount of overhead which can be undesirable in a
    high performance, high request load situation.)

Additionally, removed superfluous 'pve_' from property and method names.  The fact that they are part of the
    PVE2_API class tells us that much of their nature.  The fact that all were in protected/private scope
    makes the extra characters even more unnecessary.

Signed-off-by: Daniel Hunsaker <[email protected]>

Author: danhunsaker