Asks 2 or 4 times for the same GnuPG passphrase even if there is no passphrase (lp:#1714662)

[Original report](https://bugs.launchpad.net/bugs/1714662) created by **Valentin Stoykov (vstoykovbg)**
```
When I run duplicity with a sign key like this:

    $ duplicity --encrypt-sign-key B0ED3B59 data file:///tmp/backup1

it asks for a passphrase two times:

    GnuPG passphrase:
    GnuPG passphrase for signing key:


There is only one passphrase for this key and this should be obvious for
the duplicity.

When I add `--use-agent` it asks 4 times for a passphrase using the agent:

    $ duplicity --encrypt-sign-key B0ED3B59 data file:///tmp/backup --use-
agent


I tested it with GnuPG key with no passphrase and it still asks for a
passphrase (expected: duplicity should 'see' that there is no passphrase
for the key and don't ask for it at all).

Also, it asks 3 times for the same (empty string) passphrase when doing
incremental backup:




    valentin@computer:~/tmp$ duplicity --encrypt-sign-key FC7C18370CED0054
data file:///tmp/backup
    Local and Remote metadata are synchronized, no sync needed.
    Last full backup date: none
    GnuPG passphrase:
    GnuPG passphrase for signing key:
    No signatures found, switching to full backup.
    --------------[ Backup Statistics ]--------------
    StartTime 1504350184.04 (Sat Sep  2 14:03:04 2017)
    EndTime 1504350184.19 (Sat Sep  2 14:03:04 2017)
    ElapsedTime 0.16 (0.16 seconds)
    SourceFiles 4
    SourceFileSize 2202707 (2.10 MB)
    NewFiles 4
    NewFileSize 2202707 (2.10 MB)
    DeletedFiles 0
    ChangedFiles 0
    ChangedFileSize 0 (0 bytes)
    ChangedDeltaSize 0 (0 bytes)
    DeltaEntries 4
    RawDeltaSize 2202702 (2.10 MB)
    TotalDestinationSizeChange 2207411 (2.11 MB)
    Errors 0
    -------------------------------------------------

    valentin@computer:~/tmp$ echo test > data/test.txt
    valentin@computer:~/tmp$ duplicity --encrypt-sign-key FC7C18370CED0054
data file:///tmp/backup
    Local and Remote metadata are synchronized, no sync needed.
    Last full backup date: Sat Sep  2 14:03:01 2017
    GnuPG passphrase:
    GnuPG passphrase for signing key:
    GnuPG passphrase:
    --------------[ Backup Statistics ]--------------
    StartTime 1504350192.78 (Sat Sep  2 14:03:12 2017)
    EndTime 1504350192.79 (Sat Sep  2 14:03:12 2017)
    ElapsedTime 0.01 (0.01 seconds)
    SourceFiles 5
    SourceFileSize 2202713 (2.10 MB)
    NewFiles 2
    NewFileSize 11 (11 bytes)
    DeletedFiles 0
    ChangedFiles 0
    ChangedFileSize 0 (0 bytes)
    ChangedDeltaSize 0 (0 bytes)
    DeltaEntries 2
    RawDeltaSize 5 (5 bytes)
    TotalDestinationSizeChange 1356 (1.32 KB)
    Errors 0



Software version:

valentin@computer:~$ duplicity --version
duplicity 0.7.14
valentin@computer:~$ cat /proc/version
Linux version 4.4.0-93-generic (buildd@lgw01-03) (gcc version 5.4.0
20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4) ) #116-Ubuntu SMP Fri Aug 11
21:17:51 UTC 2017
valentin@computer:~$ cat /etc/os-release
NAME="Ubuntu"
VERSION="16.04.3 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04.3 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
VERSION_CODENAME=xenial
UBUNTU_CODENAME=xenial
valentin@computer:~$ python --version
Python 2.7.12
valentin@computer:~$
```
Original tags: gnupg passphrase

Story points: 6.0