CRM: refactor check if state transition to active is ok

CRM: refactor check if state transition to active is ok

Mainly addresses a problem where we read the manager status without
catching any possible exceptions.

As this was done only to check if our node has active fencing jobs,
which tells us that it makes no sense to even try to acquire the
manager lock - as we're fenced soon anyway.
Besides this check we always checked if we're quorate and if there
are services configured, so move
both checks in the new 'can_get_active' method, which replaces the
check_pending_fencing and the has_services method.

Move the quorum check in front and catch a possible error from the
following manager status read.
As a side effect the state transition code gets a bit shorter without
hiding the check intention.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Reviewed-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Dominik Csapak <d.csapak@proxmox.com>