[NEXUS-7581] CMA HTTP Client factory

https://issues.sonatype.org/browse/NEXUS-7581
http://bamboo.s/browse/NX-OSSF367
Note: NTLM works only if keep-alive is enabled. In NX2 there is no such check ("allow NTLM only if keep-alive enabled"), hence, it allows users to produce "invalid" config (that will never work). Wouldn't be nice to have this in NX3?