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