Use CQLSH_HOST in final call to cqlsh as well

[funny-falcon]

Which problem is this PR solving?

• docker script doesn't use CQLSH_HOST variable for final call to cqlsh
  I suppose, it makes impossible to use docker image on other host that is not part of cassandra cluster.

Short description of the changes

• add CQLSH_HOST variable in place of host argument

[codecov]

Codecov Report

Merging #1372 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1372   +/-   ##
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  Coverage     100%    100%           
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  Files         164     164           
  Lines        7376    7376           
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  Hits         7376    7376

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[funny-falcon]

Oh, I was just writting that it was wrong change :-(
I've missed that cqlsh accepts environment variable CQLSH_HOST.

My thoughts:

• default CQLSH_HOST is "cassandra"
• default CQLSH_HOST were not exported
• previously default CQLSH_HOST were not passed to final command, but it were passed to check command.

Therefore:

• if CQLSH_HOST were defined from outside (as a docker -e CQLSH_HOST argument), it worked correctly in both places (because it were passed as an argument to check command, and passed as a environment variable to final command)
• if CQLSH_HOST were not defined from outside, check command accepted default value "cassandra" as a host argument, and, probably, it failed, making whole script fail.

Therefore, correct change should be removing both usage of CQLSH_HOST as arguments.

I sorry that I sent this pull request without enough investigation.

[jpkrohling]

I kinda wondered whether cqlsh shouldn't read CQLSH_HOST, but given that we have a similar usage a few lines above (line 18), I think this is appropriate.

The default value cassandra is consistent with the docs we have, but it's really intended to be set by users, as there's no right default value we could use.