teuthology: don't use git to define version for released teuthology

teuthology: don't use git to define version for released teuthology

After installing teuthology using pip there can be found error messages
in the log when running any command, for example 'teuthology --version':

  fatal: not a git repository (or any parent up to mount point /)
  Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
  Can't get version from git rev-parse Command '['git', 'rev-parse', '--short', 'HEAD']' returned non-zero exit status 128.
  1.0.0

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@suse.com>