[Reviewer: Mike] Use find, not find_first_of

Mike, can you review this fix to use find, not find_first_of. The icscf_uri code was finding the first instance of a single character in the string 5054 in the scscf_uri and replacing it with 5052, meaning the iscsf uri for my em4 deployment was turned into sip:sprout.em5052-ngv.com:5054;transport=TCP