Changes in iSCSI CHAP tests

In rare cases, the previous rand_string function could generate a single int instead of string, which would break the test. Here it's replaced by secrets.choice, which should be as close to a good password generator in Python(not that it matters 😄). As seed, we're using all allowed characters by iSCSI standard
I'm also using this "alphabet" as default (static) username and password in CHAP tests.

Also fixing typos where most(but not all) variables were called 'tgp' or 'TGP', instead of tpg(target portal group), better to fix to avoid confusion and help with searches.

Also adding compose filter in targetcli .fmf file to enable filtering by compose. This is useful to skip CHAP tests on RHEL-7, as targetcli there does not support all characters.

Finally, adding --no-binary=stqe to README installation instructions, as it is needed to install non-package data files like /etc/multipath.conf