CLOUDSTACK-7758: Fixed although api calls are failing, event tab shows them as successful

This closes #29
CLOUDSTACK-7541: Added restriction to not allow custom disk offering with disksize UI doesn't allow but with API we were able to create custom disk offering with disk size which was causing this issue
This closes #28
CLOUDSTACK-7930, CLOUDSTACK-7931: Do not allow to set invalid values for global settings which are of type integer and float

This closes #41
CLOUDSTACK-7932: Fixed wrong semantics for isVmAlive() method in HypervInvestigator

Findbugs will report error on this as it is expecting true/false for Boolean value.
But we have diffrent meaning for null so it is false positive case from findbug

This closes #39
CLOUDSTACK-7929: While creating network offering if one specifies negative value for network rate then we will convert that value to 0 i.e. unlimited

This closes #40