Improve RPM packaging

* configure.ac: allow disabling of libuv plugin. It needed when building
  RPM on the system, where libuv-devel *is* available. Also do not fail
  when wget/curl was not found, because probably they will not needed at
  all. In any case further task (like mock or gtest, will double check
  that dowloader is accessible).

* packaging/rpm/libcouchbase.spec.in:  add openssl-devel to build
  dependencies, and pass option to ./configure script to disable
  libuv. Also small typo was fixed in the changelog section.

* packaging/rpm/package.mk: define that we are using md5 algorithm for
  hashsums, with that given all system can unpack and build the
  binaries (at least on COPR)

Change-Id: I18709c54ef3b100a7b9007749cb88fcbbb779c7b
Reviewed-on: http://review.couchbase.org/40683
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>