add sftp support

[cortex93]

Related to #265

Currently centos 7 is broken

Step 58/64 : RUN         DIR=/tmp/ffmpeg && mkdir -p ${DIR} && cd ${DIR} &&         ./configure         --disable-debug         --disable-doc         --disable-ffplay         --enable-shared         --enable-avresample         --enable-libopencore-amrnb         --enable-libopencore-amrwb         --enable-gpl         --enable-libass         --enable-fontconfig         --enable-libfreetype         --enable-libvidstab         --enable-libmp3lame         --enable-libopus         --enable-libtheora         --enable-libvorbis         --enable-libvpx         --enable-libwebp         --enable-libxcb         --enable-libx265         --enable-libxvid         --enable-libx264         --enable-nonfree         --enable-libssh         --enable-openssl         --enable-libfdk_aac         --enable-postproc         --enable-small         --enable-version3         --enable-libbluray         --enable-libzmq         --extra-libs=-ldl         --prefix="${PREFIX}"         --enable-libopenjpeg         --enable-libkvazaar         --enable-libaom         --extra-libs=-lpthread         --enable-libsrt         --enable-libaribb24         --extra-cflags="-I${PREFIX}/include"         --extra-ldflags="-L${PREFIX}/lib" &&         make &&         make install &&         make tools/zmqsend && cp tools/zmqsend ${PREFIX}/bin/ &&         make distclean &&         hash -r &&         cd tools &&         make qt-faststart && cp qt-faststart ${PREFIX}/bin/
 ---> Running in c261a1eaee03
ERROR: aom >= 1.0.0 not found using pkg-config

don't know how my changes can affect aom.

I add libssh to base image in addition to libssh-dev to build image but it may not be necessary to.