Commands for managing static IP addresses

Added commands:
azure vm static-ip show <vm-name>
azure vm static-ip set <vm-name> <ip-address>
azure vm static-ip remove <vm-name>
azure network vnet static-ip check <vnet> <ip-address>

Added --static-ip option to:
azure vm create
azure vm docker create

@babumuralidharan it seems that all bugs on this feature have been addressed.
@avranju , please review. This PR is assuming we want to merge into MSOT/dev and not send directly upstream.
Good catch! I agree that some validation would be desirable here. It seems like a general problem in the code base (with the exception of Mobile._js, see promptIfNotGiven wrapper there), that whenever we call promptIfNotGiven, we don't validate non-empty return values. I am thinking:
a) We should modify promptIfNotGiven itself to either keep asking or throw an error if the user just presses the enter key.
b) This warrants a separate issue and PR.
What do you think?