scripts: fix shellcheck warnings

[tiborsimko]

$ shellcheck ./scripts/create-admin-user.sh

In ./scripts/create-admin-user.sh line 25:
echo $REANA_DB
     ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
echo "$REANA_DB"


In ./scripts/create-admin-user.sh line 38:
                     flask reana-admin create-admin-user --email $email_address --password $password)
                                                                 ^------------^ SC2086: Double quote to prevent globbing and word splitting.
                                                                                           ^-------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
                     flask reana-admin create-admin-user --email "$email_address" --password "$password")


In ./scripts/create-admin-user.sh line 39:
if [ $? -ne 0 ]; then
     ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.


In ./scripts/create-admin-user.sh line 41:
    echo $admin_access_token
         ^-----------------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
    echo "$admin_access_token"

For more information:
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2181 -- Check exit code directly with e.g...