[devstack] shellcheck reports a lot of errors over devstack-plugin plugin.sh

$ shellcheck plugin.sh -f gcc | wc -l
     168

$ shellcheck plugin.sh -f gcc | cut -d : -f 4- | sort | uniq -c
 156 note: Double quote to prevent globbing and word splitting. [SC2086]
   2 note: Use $(..) instead of legacy `..`. [SC2006]
   1 note: read without -r will mangle backslashes. [SC2162]
   2 warning: Declare and assign separately to avoid masking return values. [SC2155]
   1 warning: MURANO_PYTHONCLIENT_DIR appears unused. Verify it or export it. [SC2034]
   1 warning: Possible misspelling: MURANO_USE_GLARE may not be assigned, but murano_use_glare is. [SC2153]
   1 warning: Quote the rhs of = in [[ ]] to prevent glob matching. [SC2053]
   2 warning: Quote this to prevent word splitting. [SC2046]
   1 warning: Use cd ... || exit in case cd fails. [SC2164]
   1 warning: horizon_config_part appears unused. Verify it or export it. [SC2034]

These are mostly trivial errors and should be easy to fix.