kernel: use utsrelease string for need_to_install() purposes

Currently, to see if a node has rebooted into the right kernel,
need_to_install() compares a given 40-char commit hash with a 7-char
commit hash abbreviation it pulls from the output of 'uname -r'.

gitbuilders can now export UTS_RELEASE kernel version string through
.../$SHA1/version file.  Use this string instead of the 40-char commit
hash and compare it with the output of 'uname -r' directly.  This saves
us the parsing exercise and, more importantly, makes it possible to
install clean tagged kernels using 'tag:' element, which wasn't
possible before because versing string of such kernels doesn't have
a commit hash in it.

If version file is unavailable, fallback to the existing way of doing
things.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>